Crop Factor & Magnification factor


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johngiam

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#1
1. First things first, I always heard from my seniors that the real image distance is lens distance multiply by crop factor.

Meaning: 50mm prime * 1.5 crop (d80) gives 75 mm

Does it mean I fix a 50mm prime but I am shooting at 75 mm?

2. Now I played around that distance and estimated on and off the viewfinder to find 50mm on my d80 viewfinder is about the same size as the real image. So why that difference between a 50mm and 75 mm when visual on and off the viewfinder it looks the same?

3. Another issue following up to the first 2 qns, so the seniors who told me about this distance *crop factor value told me, that is why I shoot an object through a viewfinder, its slightly different on what would appear on the preview lcd.

meaning: I zoom in to a petal of a flower, but the photo previewed is "expanded" outwards a little such that i see another petal.

Just before we stick this problem to crop factor again, i like to share something. I've done a little read up of d80 in dpreview and I found out it has a 0.95 viewfinder magnification factor. I find this magnification factor a more logical reason why my photos are "expanded" out wards a bit rather than the crop factor again.

Thus, I like a clear cut clarification between crop factor and magnification factor.

Cheers and thanks in advance! =)
 

2ichigo2

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Jul 18, 2007
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#2
1. When talking about crop factor is comparing with full frame camera body...
You can only tell the difference when u compare pics taken by same lens with 1.5crop factor body and full frame body.

Example:
50mm in D80 (75mm due to crop factor): the pic size [ ]
50mm in D3: the pic size [ ]

2. As for viewfinder, in the manual it will state that the viewfinder only see certain percentage of what u will be taking of the pic. If im not wrong 95% for 400D. Which means that when i view thru the viewfinder, i only see 95% of what the photo is to be taken. Example when i see thru viewfinder but i cant spot the lamp-post at the most right side but after taking the pic, the extra 5% actually captures the lamp-post.

3. As for magnification factor is more for macro-photography. Lens with 1:1 magnification factor means that it can take life size object. To me in simple explaination, 1:1 can take shots of small ant filling up the whole photo without cropping. 1:2 will make the small ant filling up half of photo. Where as special macro lens that magnifies example 1:2x can take shots of the small ant's head and fill up the entire photo.

1:1 [ANT] - 1:2 [ ant ] - 1:2x [ N ]

Hope my nub explaination can help u~
 

johngiam

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#3
No nub reply. I enlightened !

But I still don get what you mean in answer #1. Do you mean, I use 50mm on FF i see the real 50mm distance but in d80 i see slight larger than 50mm?
 

giantcanopy

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Feb 11, 2007
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#5
1. First things first, I always heard from my seniors that the real image distance is lens distance multiply by crop factor.
Thats an incorrect explaination. The focal length of the lens remains the same whatever the crop factor. And wats this thing about real image distance in the first place?

Does it mean I fix a 50mm prime but I am shooting at 75 mm?
Nope you are still shooting with a 50mm prime lens, the focal length being fixed ..

The field of view of a 50mm lens on your cropped digital sensor looks like that of a 75mm on a 35mm film / sensor

2. Now I played around that distance and estimated on and off the viewfinder to find 50mm on my d80 viewfinder is about the same size as the real image. So why that difference between a 50mm and 75 mm when visual on and off the viewfinder it looks the same?
What you are seeing thru the viewfinder is approximated to that of a 75mm. ( The picture u take will seem a little wider than the viewfinder because the D80 viewfinder can show u approx 0.95x of what u will be taking. )

Of course if ur camera is a 35mm sensor / film, what you see in that viewfinder will seem wider

3. Another issue following up to the first 2 qns, so the seniors who told me about this distance *crop factor value told me, that is why I shoot an object through a viewfinder, its slightly different on what would appear on the preview lcd.
That will be the 0.95x that u are mentioning :) The LCD displays the entire shot captured, including that 0.05x bit of extra image that u cannot see thru the viewfinder.

Ryan
 

lsisaxon

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Nov 29, 2004
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#6
1. First things first, I always heard from my seniors that the real image distance is lens distance multiply by crop factor.

Meaning: 50mm prime * 1.5 crop (d80) gives 75 mm

Does it mean I fix a 50mm prime but I am shooting at 75 mm?

2. Now I played around that distance and estimated on and off the viewfinder to find 50mm on my d80 viewfinder is about the same size as the real image. So why that difference between a 50mm and 75 mm when visual on and off the viewfinder it looks the same?

3. Another issue following up to the first 2 qns, so the seniors who told me about this distance *crop factor value told me, that is why I shoot an object through a viewfinder, its slightly different on what would appear on the preview lcd.

meaning: I zoom in to a petal of a flower, but the photo previewed is "expanded" outwards a little such that i see another petal.

Just before we stick this problem to crop factor again, i like to share something. I've done a little read up of d80 in dpreview and I found out it has a 0.95 viewfinder magnification factor. I find this magnification factor a more logical reason why my photos are "expanded" out wards a bit rather than the crop factor again.

Thus, I like a clear cut clarification between crop factor and magnification factor.

Cheers and thanks in advance! =)
There is a lot to the optics if you really want to know the magnification factor at each stage of the optical system.

The VF has it's own magnification, the lens has it's own, and then there's a crop factor.

What has changed? If you think about it, what has really changed is just the size of the image sensor. Previously for film, a 50mm would give 46 degrees angle of coverage. Now you have reduced the size of your frame, you would then be only taking a portion (crop) of the original 46 degrees. The angle has to be smaller. And this is equivalent to the angle as if you had used a lens that is 1.5x the focal length on film which give you 31.5 degrees.

Does it matter how big it appears in your viewfinder? No, your image is composed based on the entire frame. On a D70 series body, the viewfinder magnification is smaller, D80's is bigger. But the picture angle you get from the same lens across the frame would still be the same.

Now if we look at the LCD, different cameras also have different LCD sizes, you can also zoom your image to any magnification, the sensor captures in different resolution, etc.. All these will affect the size of your image. Does it matter? In my opinion, no. My image is still relative to the frame size. The same frame, I can print 4R or Super 8R, magnification will be different, but does it matter? I don't think so. My image is still relative to the frame size.

It's like watching TV with a 6" or a 54", you still get the entire frame, just the size is different. So you cannot simply look in the viewfinder and say "Oh if I use a 75mm, it looks life size." Different bodies have different viewfinder magnification.
 

StrifeYun

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Nov 15, 2006
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#7
alot of threads on this .... :p

wonder why they call it crop factor. (i still find this a marketing scam hahahaha)

1.5 crop factor = you will capture 1.5 times SMALLER image AREA on the sensor, as compare to a Full Frame (crop factor of 1.0).
 

johngiam

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#8
Thanks chongkm, forgot this incredible wiki aha

There is a lot to the optics if you really want to know the magnification factor at each stage of the optical system.

The VF has it's own magnification, the lens has it's own, and then there's a crop factor.

What has changed? If you think about it, what has really changed is just the size of the image sensor. Previously for film, a 50mm would give 46 degrees angle of coverage. Now you have reduced the size of your frame, you would then be only taking a portion (crop) of the original 46 degrees. The angle has to be smaller. And this is equivalent to the angle as if you had used a lens that is 1.5x the focal length on film which give you 31.5 degrees.

Does it matter how big it appears in your viewfinder? No, your image is composed based on the entire frame. On a D70 series body, the viewfinder magnification is smaller, D80's is bigger. But the picture angle you get from the same lens across the frame would still be the same.

Now if we look at the LCD, different cameras also have different LCD sizes, you can also zoom your image to any magnification, the sensor captures in different resolution, etc.. All these will affect the size of your image. Does it matter? In my opinion, no. My image is still relative to the frame size. The same frame, I can print 4R or Super 8R, magnification will be different, but does it matter? I don't think so. My image is still relative to the frame size.

It's like watching TV with a 6" or a 54", you still get the entire frame, just the size is different. So you cannot simply look in the viewfinder and say "Oh if I use a 75mm, it looks life size." Different bodies have different viewfinder magnification.
Thank you so so much for taking time to answer all my question specifically! wiki plus ur answers helped me realise more!

alot of threads on this .... :p

wonder why they call it crop factor. (i still find this a marketing scam hahahaha)

1.5 crop factor = you will capture 1.5 times SMALLER image AREA on the sensor, as compare to a Full Frame (crop factor of 1.0).
no "magnification" involved here.
Thanks, that was a short and sweet explanation, confirmed my findings as well!

Finally understand better that crop factor gives the same zoom distance but a narrow angle of what one can see through a crop factor!
 

lsisaxon

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Nov 29, 2004
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#9
alot of threads on this .... :p

wonder why they call it crop factor. (i still find this a marketing scam hahahaha)

1.5 crop factor = you will capture 1.5 times SMALLER image AREA on the sensor, as compare to a Full Frame (crop factor of 1.0).
Because it is a crop factor. When you crop, it appears like you have used a longer focal length lens with a reduced angle of view. In film days, I always shoot a little bit wider and then crop for composition during printing.
 

justanut

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Dec 4, 2007
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#10
I've always wanted to write something about this crop factor things as I've noticed many newbies like myself getting confused over it.

Most notably questions like "Will the 50mm f/1.2 on my 1.6x camera give me the same effect/fov/output as the 85 f/1.2 on the FF camera?"

The answer is yes and no.

Yes, you will have the same FOV, ie. If you use the 85mm on a FF camera and take a group photo with the whole group just managing to squeeze into the frame, if you do not move and switch to a 1.6x camera with the 50mm you will similarly have the whole group filling up the frame.

No as in the background compression will be different. The 85 will show less of the visible background while the 50 will show more of the visible background. ie. If with the 85mm on a FF camera you are able to see 3 trees in the background of the group photo, you might see 6 or 7 trees in the background with the 50mm instead. Depending on the distance the trees are away of course.

So you will not get the same milky smooth bokeh with the 50mm on a crop compared to the 85mm on a FF!

I think there are websites that illustrate this with photos much better than I can explain.

Lastly, why it is called a crop factor is simply because in photoshop you CROP the photo to cut out what you do not want. Imagine taking a picture with the 50mm on a FF camera at the same distance as the above example. You would definitely have more space to the left and right of the group, the 50mm being "wider". When you CROP this picture in photoshop to frame the picture similar to what the 85mm gave you, you're in fact replicating what the 50mm on the 1.6x camera does. It CROPS your pictures.

Was that helpful at all? I wonder... :sweat:
 

#12
I started photography at the digital age, so these talks about crop factor, field of view really means nothing to me. Personally, I do not care about them, as fas as I'm concerned, I frame the shot in a manner that I like and take the shot in the focal length that makes sense to me. I know that a 200mm lens can allow me to take a shot at the distant lion in the zoo, that's all I'm concerned about.

What I'm trying to say is - don't worry about crop factor... well, it is good to know about it and how the name 'crop factor' comes about (on hindsight, the person who even 'invented' this term ought to be shot for creating this 'confusion') , but other than that, if it doesn't make sense to you, then you really don't need to lose sleep or worry over it. Just concentrate on composing and taking pictures with your computer.... erm... I mean DSLR. ;p

Just my 2 cents worth....
 

Apr 12, 2005
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#14
No as in the background compression will be different. The 85 will show less of the visible background while the 50 will show more of the visible background. ie. If with the 85mm on a FF camera you are able to see 3 trees in the background of the group photo, you might see 6 or 7 trees in the background with the 50mm instead. Depending on the distance the trees are away of course.

I don't know whether you're talking about
85mm on FF vs 50mm on a 1.6x crop factor sensor
or
85mm on FF vs 50mm on FF.

What you're describing is actually the Field of View (something which you have just mentioned earlier in your post) and perspective.

Field of view is about what is included or excluded within the picture frame and perspective is about the relative sizes of things in the picture.

Perspective is only determined by shooting distance (if we ignore lens distortion such as those in wide angle).

50mm on a 1.6x crop factor sensor will have exactly the same Field of view and perspective as 85mm on FF if both are taken from the same shooting position. This means the background compression will be the same and what is included in the background will also be the same. If 3 trees are seen with the 85mm on FF, then the same 3 trees will be seen with 50mm on 1.6x crop factor sensor. If the tree in front appears is 1.5x taller than the tree behind it on the picture taken by 85mm on FF, then the same tree will also appear 1.5x taller than the tree behind it on the picture taken by 50mm on 1.6x crop factor sensor as long as the shooting position is the same regardless of focal length. This can be mathematically shown by using the magnification factor. However, I shall not bore people with the maths.

This is also why a picture taken at about 6mm on a 1/2.5" sensor (nearly 6x crop factor compared to FF) on a compact camera will be almost the same (foreground as well as what is included in the background) as 35mm taken on a FF and about 6-18mm is described in many compact cameras' specifications as 35-105mm equivalent.

The main difference is of course Bokeh and Depth of Field since sensor size and actual focal length are different.
 

kurtlim

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Apr 30, 2003
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#15
Just before we stick this problem to crop factor again, i like to share something. I've done a little read up of d80 in dpreview and I found out it has a 0.95 viewfinder magnification factor. I find this magnification factor a more logical reason why my photos are "expanded" out wards a bit rather than the crop factor again.

Thus, I like a clear cut clarification between crop factor and magnification factor.
viewfinder magnification factor has nothing to do with FOVCF as you can read from Wikipedia. when the viewfinder magnification factor is 1x, what you see thru the viewfinder will look exactly the same size as you would with your naked eye--provided you are using the normal lens--~50mm on 35mm camera. meaning with a standard lens mounted, you could look through the viewfinder with one eye and see the rest of the scene with the other eye and both will look the same size--stereo vision. with 0.9x magnification factor, what you see from the viewfinder will look smaller than real world--again, with standard lens.


I started photography at the digital age, so these talks about crop factor, field of view really means nothing to me. Personally, I do not care about them, as fas as I'm concerned, I frame the shot in a manner that I like and take the shot in the focal length that makes sense to me. I know that a 200mm lens can allow me to take a shot at the distant lion in the zoo, that's all I'm concerned about.

What I'm trying to say is - don't worry about crop factor... well, it is good to know about it and how the name 'crop factor' comes about (on hindsight, the person who even 'invented' this term ought to be shot for creating this 'confusion') , but other than that, if it doesn't make sense to you, then you really don't need to lose sleep or worry over it. Just concentrate on composing and taking pictures with your computer.... erm... I mean DSLR. ;p

Just my 2 cents worth....
they do matter, unless you never change lens, like in PnS camera; or you never use 35mm lens. blame the manufacturer who cant make a full frame 35mm sensor affordable.
 

justanut

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#16
Thats not what I see leh... The lens is still the same lens with the same angle. Using 50mm on a 1.6x isn't going to give u the same background as the 85mm on a FF. Maybe you should try it out...?

Thats also the reason why for us Canon users the 50/1.2L on the 40D is not going to give you the same result as the 85/1.2L on the 5D i think?

What I want to say is that the 1.6x isn't actually the same as using an multiplier extender (which is why your aperture doesn't drop...) Its simply to make it easier for us to see what kind of FOV we'll get. Perspective however remains the same, depending on the lens. Using a 85 on a 1.6 and a FF will give you the same background distortion (bokeh). The sensor just can't take in as much so it CROPS the centre part out.

Aiyo... this damn thing confuses all the time...

I don't know whether you're talking about

This is also why a picture taken at about 6mm on a 1/2.5" sensor (nearly 6x crop factor compared to FF) on a compact camera will be almost the same (foreground as well as what is included in the background) as 35mm taken on a FF and about 6-18mm is described in many compact cameras' specifications as 35-105mm equivalent.

The main difference is of course Bokeh and Depth of Field since sensor size and actual focal length are different.
 

Apr 12, 2005
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#17
Thats not what I see leh... The lens is still the same lens with the same angle. Using 50mm on a 1.6x isn't going to give u the same background as the 85mm on a FF. Maybe you should try it out...?

Thats also the reason why for us Canon users the 50/1.2L on the 40D is not going to give you the same result as the 85/1.2L on the 5D i think?

What I want to say is that the 1.6x isn't actually the same as using an multiplier extender (which is why your aperture doesn't drop...) Its simply to make it easier for us to see what kind of FOV we'll get. Perspective however remains the same, depending on the lens. Using a 85 on a 1.6 and a FF will give you the same background distortion (bokeh). The sensor just can't take in as much so it CROPS the centre part out.

Aiyo... this damn thing confuses all the time...
I don't quite get you when you say "The lens is still the same lens with the same angle".

Field of view (FOV) is the result of not only lens but also sensor size. So what you can see (i.e. included) in the background depends on the focal length, sensor size and of course, your shooting distance.

A picture speaks a thousand words :



If a person has 3 cameras : a FF (e.g. Canon 5D) with 80mm, a 1.6x crop camera (e.g. Canon 40D) with 50mm and a compact camera at about 14mm and then shoot the same subject from the same position with the 3 cameras, he will end up with the same picture with the same composition and perspective. The FOV on the 3 cameras will be the same and therefore whatever is included in the background will be the same.

The above answers threadstarter's question. When you're shooting with 50mm on the Nikon D80, your focal length is still 50mm but because the image sensor is smaller than a FF, the field of view is the same with 75mm on a FF and, for the Canon 40D, the FOV is the same as 80mm on FF.

As said in the earlier post, what is included in the background will be the same for 35mm on FF, 22mm on 1.6x crop factor camera and about 6mm on a compact camera because all have the same 35mm equivalent field of view.

Canon users with the 50/1.2L on the 40D will get a slightly wider FOV than the 85/1.2L on the 5D because 50mm on the 40D has the same FOV as 80mm on the 5D. If 80mm is used on the 85/1.2L for 5D, then the FOV (foreground and background) will be the same as 50mm on the 40D. This are of course differences in Depth of Field and Bokeh as a result of the combination of actual focal length, sensor size and lens construction.

Using an extender (i.e. teleconverter) of course will result in a narrower FOV, resulting in less in the background seen because when you add an extender on a camera, the focal length is increased without any change in the sensor size but a longer focal length has a narrower lens angle of view. (Just sidetracking, the decrease in aperture F number when adding an extender is because F number is the diaphragm size divided by the focal length and so when focal length is increased with an extender without any change in diaphragm, the F number becomes larger in the denominator ……e.g. F/4 becomes 1/(4x2) =F/8 when a 2x extender is added. This is related to light lost for exposure as focal length becomes longer and it’s a little complicated but suffice to say that the total amount of light going into the lens is decreased proportionally but the diaphragm is unchanged as the FOV becomes narrower and the less light coming in through the lens get spreaded onto the same sensor surface area such that when a 2x converter is used, what was originally F/4 exposure becomes equivalent to an F/8 exposure).


Again a picture speaks a thousand words (for e.g. focal length is increased from 50mm to 80mm either with a longer lens or by a 1.6x teleconverter (assuming there is one although the norm is 1.4x and 2x):



In contrast, when the focal length is increased from 50mm to 80mm and the sensor size changes from a 1.6x crop factor to a FF, then narrower angle of a longer focal length lens will be exactly offset by the wider angle in the sensor larger by the same 1.6x factor and thus, the FOV remains the same.



In summary, 80mm on FF (e.g. Canon 5D) will be the same as 50mm on a 1.6x crop factor camera (e.g. Canon 40D) as far as foreground and background are concerned as long as they're shot from the same position to have the same picture and composition. Indirectly, it also means 80mm on the 5D has a wider FOV than 80mm on the Canon 40D and therefore more can be seen not only in the background but also foreground if shot from same position since the Canon 5D has a larger image sensor or just more in the background if standing nearer by a factor of 1.6x so that the foreground is the same with 80mm on the Canon 40D. 80mm on the Canon 40D afterall has a narrower FOV equivalent to 80x1.6 = 128m on the 35mm format on the FF.



Perspective in strict photographer terms means how relative distances and sizes are rendered in the picture and it is entirely determined by shooting distance and contrary to popular belief, not affected by focal length or sensor size. As I have said in my earlier post, it has to do with magnification factor.

If one object is 2m behind another and you shoot from 5m away,


If you use 50mm, your magnfication factors will be :

1st object which is 5m away : 50mm/5m = 50/50,000 = 1/1000
2nd object which is 7m away : 50mm/7m = 50/70,000 = 1/1400

So 1/1000 vs 1/1400 is 1400 : 1000 = 7 : 5
= 1.4 : 1 relative magnification size ratio

If you use 200mm, your magnificator factors will be :
1st object which is 5m away : 200mm/5m = 200/50,000 = 1/250
2nd object which is 7m away : 200mm/7m = 200/70,000 = 1/350

1/250 : 1/350 = 3.5 : 2.5 = &:5
= 1.4 : 1 (Same as earlier with 50mm lens.)

So if you shoot from the same position, the relative sizes of things in the picture will always be the same regardless of what focal length you use. So the perspective is the same for different focal length as long as the shooting distance is the same. The difference is only in the FOV (i.e. what is included or excluded in the foreground and background). Again, Bokeh and DOF are separate issues.

The confusion in perspective comes about because people stand at different distances with different focal lengths. Usually, longer focal length is used at much further shooting distance and as a result, distances seem to be compressed as relative size differences become less pronounced. This is caused by the further distance and, contrary to popular misconcepton, not by the focal length. Again, this can be shown mathematically :

Now instead of standing at 5m away in the earlier example, now stand at 10m away.

1st object which is 10m away : 50mm/10m = 50/100,000 = 1/2000
2nd object which is 12m away : 50mm/12m = 50/120,000 = 1/2400

So 1/2000 vs 1/2400 is 2400 : 2000 = 1.2 : 1 relative magnification size ratio

And the 2nd object is now bigger relative to the 1st object than when shooting at 5m away and therefore distance appears compressed. This can be easily seen that 2m difference as a % of 5m is greater than 2m difference as a % of 10m.

If the above object 1 and 2 are the same size, then the 2nd object will appear 1/1.4 = 0.71x the size of the 1st object for the first picture at 5m away. But when shot from 10m away, the 2nd object will apppear to be 1/1.2 = 0.83x the size of the 1st object. Of course in absolute terms, the 2 objects will appear smaller in the 2nd picture at 10m away than the 1st picture at 5m away.

If you stand very very far away such as 500m away, then the ratio would become almost 1:1 and the 2m distance difference between the 2 objects would appear almost nothing in the picture and they look like they're just next to each other and if they're same real physical size, then will appear the same size in the picture. This is true even if you use a ultra wide angle lens such as 12mm. That is why very distant objects appear next to each other in many landscape pictures even though the focal lengths used are very short while the distance in the foreground which is very near becomes very exaggerated.

To see that perspective is the same at 10m away for a 200mm lens and a 50mm lens :
If you use 200mm, your magnification factors will be :
1st object which is 10m away : 200mm/10m = 200/100,000 = 1/500
2nd object which is 7m away : 200mm/12m = 200/120,000 = 1/600

1/500 : 1/600 = 1.2 : 1
Same as earlier with 50mm lens. So perspective is the same again with different focal lengths as long as it's shot from the same distance (10m in this case).
 

lsisaxon

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Nov 29, 2004
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#18
Thats not what I see leh... The lens is still the same lens with the same angle. Using 50mm on a 1.6x isn't going to give u the same background as the 85mm on a FF. Maybe you should try it out...?
50mm on a 1.6x crop is still wider than a 85mm on 135 format. 50x1.6=80.
 

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