Lense and hyperfocal distance


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Jun 1, 2004
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#1
I posted this question in Nikon section. As it's related to general photography, so I think it would be good to post it here as well.

I've a few questions regarding DX lenses and hyperfocal distance.

1. As I know DX lenses are designed for APS sensor on Nikon dslr. I'm using Nikon D40. When looking at the focal length scale on the lense, do I still need to multiple that with 1.5? Or the focal length on the lense has been adjusted for 1.5 magnification factor? e.g. When I turn to 35mm on the lense, does it mean the actual focal length is 35 * 1.5 = 52.5mm? or this 35mm is already the actual focal length?

2. This question is actually related to the 1st question. I've a hyperfocal distance chart based on nikonians.org that shows for me all hyperfocal focus points for a given focal length and aperture. According to the chart, at 35mm and f/11, the hyperfocal distance is 5.45m. My question is should I turn the lense to 35mm or 52.5mm (35mm * 1.5) for this hyperfocal distance?

3. This question was brought forward by another forum member. For "1/focal length rule" to prevent image blur because of camera shake, should we multiply the focal length on the lense with 1.5?
 

Andy Ang

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Jan 10, 2006
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#2
I posted this question in Nikon section. As it's related to general photography, so I think it would be good to post it here as well.

I've a few questions regarding DX lenses and hyperfocal distance.

1. As I know DX lenses are designed for APS sensor on Nikon dslr. I'm using Nikon D40. When looking at the focal length scale on the lense, do I still need to multiple that with 1.5? Or the focal length on the lense has been adjusted for 1.5 magnification factor? e.g. When I turn to 35mm on the lense, does it mean the actual focal length is 35 * 1.5 = 52.5mm? or this 35mm is already the actual focal length?

2. This question is actually related to the 1st question. I've a hyperfocal distance chart based on nikonians.org that shows for me all hyperfocal focus points for a given focal length and aperture. According to the chart, at 35mm and f/11, the hyperfocal distance is 5.45m. My question is should I turn the lense to 35mm or 52.5mm (35mm * 1.5) for this hyperfocal distance?

3. This question was brought forward by another forum member. For "1/focal length rule" to prevent image blur because of camera shake, should we multiply the focal length on the lense with 1.5?
1.) Theoretically, it is still 35mm. But because of the crop factor, what is going to happen on your image is going to be similar to 52mm. So I guessed the answer you are looking for is: 52.5mm.

2.) Turn according to the hyperfocal distance listed. Without the crop factor. But then again, I doubt the effectiveness of DX lens with respect to hyperfocal distances. But then again, it is juz personal.

3.) The Rule is only a rule of thumb. THere have been plenty of argument. I advise you not brood about it. For me, I can work slower then the rule. You may or may not.
 

ST1100

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Jun 18, 2003
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#3
1. As I know DX lenses are designed for APS sensor on Nikon dslr. I'm using Nikon D40. When looking at the focal length scale on the lense, do I still need to multiple that with 1.5? Or the focal length on the lense has been adjusted for 1.5 magnification factor? e.g. When I turn to 35mm on the lense, does it mean the actual focal length is 35 * 1.5 = 52.5mm? or this 35mm is already the actual focal length?

2. This question is actually related to the 1st question. I've a hyperfocal distance chart based on nikonians.org that shows for me all hyperfocal focus points for a given focal length and aperture. According to the chart, at 35mm and f/11, the hyperfocal distance is 5.45m. My question is should I turn the lense to 35mm or 52.5mm (35mm * 1.5) for this hyperfocal distance?
The hyperfocal length on the lenses are marked for full-frame. All things equal, yes, you need adjust the hyperfocal distance for a smaller sensor.

Unfortunately the hyperfocal distance has to be recomputed to factor in the different sensor size.

As an example:
The hyperfocal distance (FD) for a 50mm @f11 for fullframe is 9m.
The FD for a 75mm @f11 for fullframe is 20m.
The FD for a 50mm @f11 for a 1.5x crop is ~13m.

Since using 9m x 1.5 would give you 13.5m, i would say multiplying the FD by the distance given by the lens by the crop factor would get you the closest approximation.
 

theRBK

Senior Member
May 16, 2005
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#4
I posted this question in Nikon section. As it's related to general photography, so I think it would be good to post it here as well.
I think you should just post in one section, the general discussions one...

I've a few questions regarding DX lenses and hyperfocal distance.

1. As I know DX lenses are designed for APS sensor on Nikon dslr. I'm using Nikon D40. When looking at the focal length scale on the lense, do I still need to multiple that with 1.5? Or the focal length on the lense has been adjusted for 1.5 magnification factor? e.g. When I turn to 35mm on the lense, does it mean the actual focal length is 35 * 1.5 = 52.5mm? or this 35mm is already the actual focal length?
the focal length of the lens does not change... what changes is what you can see, and what registers on the camera's imaging sensor, is an angle of view that is smaller than if such a lens were to be used on a so called full frame camera... what the so called focal length multiplier is supposed to describe is the angle of view that a lens is reduced to when mounted on a DX camera eg. a 35mm lens, will still still be a 35mm lens, but what registers in a DX camera is a view equivalent to the angle of view that is seen on a 52.5mm lens mounted on a so called full frame camera...

2. This question is actually related to the 1st question. I've a hyperfocal distance chart based on nikonians.org that shows for me all hyperfocal focus points for a given focal length and aperture. According to the chart, at 35mm and f/11, the hyperfocal distance is 5.45m. My question is should I turn the lense to 35mm or 52.5mm (35mm * 1.5) for this hyperfocal distance?
accordingly, the depth of field of a 35mm lens mounted on a DX camera will still be the same depth of field (actually there is a slight diff, but it is mostly ignorable and depends more on how big you print, if you print really big) as if the lens were mounted on a so called full frame camera... when people are saying that DX has more depth of view than so called full frame, that is when they compare a 75mm lens with a 50mm lens that has the angle of view of a 75mm lens on a DX camera, when obviously when shooting at the same distance the 50mm camera would definitely have more depth of view than a 75mm lens...

3. This question was brought forward by another forum member. For "1/focal length rule" to prevent image blur because of camera shake, should we multiply the focal length on the lense with 1.5?
this is more of a guide than a rule... and at long telephoto distances, whether on a so called full frame or a so called DX format, it is good to multiply the required shutter speed by about 1.5...
 

Jun 1, 2004
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#5
Initially I posted it in Nikon section as it's related to DX lens but later I feel that this is also applicable to other users as well. My apology for that and thanks a lot for the answer.

I did a lot of readings on this subject these few days and came up with a conclusion. Correct me if I'm wrong.

Focal length is defined as "distance from the middle of the lens to its focal point". So the distance from from the middle of the lens to the camera's sensor is same regardless of whatever camera you use, dslr or 35mm. If you are using 50mm on a 35mm camera, the focal length would also be 50mm when you use it on dslr as the distance from the middle of the lens to the sensor is the same.

In regards to the sensor size, what actually change is the field of view (FOV), not the focal length. The dslr sensor is smaller than 35mm, basically a crop of 35mm. So your focal length will only change if you want to achieve the same FOV.

The conclusion is regardless of whatever camera you use, no changes are needed for the hyperfocal distance chart or 1/focal length rule.

You can get to the discussion on the Nikon section below.
http://forums.clubsnap.org/showthread.php?p=3018400&posted=1#post3018400
 

ST1100

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Jun 18, 2003
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#7
I did a lot of readings on this subject these few days and came up with a conclusion. Correct me if I'm wrong.

In regards to the sensor size, what actually change is the field of view (FOV), not the focal length. The dslr sensor is smaller than 35mm, basically a crop of 35mm. So your focal length will only change if you want to achieve the same FOV.

The conclusion is regardless of whatever camera you use, no changes are needed for the hyperfocal distance chart or 1/focal length rule.

Hi Desmond.

Sorry, i don't know what you've been reading up, but if you've gone into the basic definitions of DOF and hyperfocal distance, you'd realize that the hyperfocal distance MUST change when using a crop factor.

Briefly, the idea of a 'depth of field' refers to a distance wherein subjects are considered 'in focus', when printed at a certain size.

The degree of blur (technically circle of confusion, or 'COC') is set by the desired degree of enlargement from film to print.

When that film (sensor) is reduced in size, the amount of enlargement increases, and for the same level of acceptable sharpness on the same size print, the sharpness requirement on the film is increased. In other words, for the same level of sharpness on the same print size, a 1.5x crop camera needs to be (loosely speaking) 1.5x sharper.

The hyperfocal distance for the lens of a certain focal length changes from a FF to a 1.5x crop, not because the focal length has changed, but because the format (ie 1.5x crop is a smaller format than FF) has changed, and hence the sharpness requirement has changed. (This is also the reason why large format films are used for landscaping: when the film size is enormous, the degree of enlargement is much lower, and hence a much more forgiving hyperfocal distance.)
 

blueayz

New Member
Nov 19, 2004
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#8
I am also new to this and I'd like to share my own understanding and also to ask a slightly related question, if TS would allow me to share his thread.

Firstly, I agree with the TS on the issue of focal length and fov. The focal length of the lens does not change with the size of the sensor. Hence the optical magnification of the lens also does not change. What changes is the fov of the lens.

Hence in the example given of a 35mm lens, the magnification of the lens is the same no matter if it used on a full frame sensor or an APS sensor. But the field-of-view of the APS sensor will be equivalent to that of a 52.5mm lens, meaning it is not able to capture the full image circle generated by the 35mm lens, and leaves a border unable to be captured by the small sensor. The image below tries to illustrate this.



And as most lenses have some slight distortion and softness at the edges of the image, an FF sensor would give a different perspective, whereas an APS sensor would only take the best part of the lens image, the centre part, hence less noticeable distortion and no edge softness. I believe this also applies to the 4/3 sensor. This is how I understand it so far.


And my question is:
What is the difference between hyperfocal focus and infinity focus. Where does hyperfocal focus distance stop and infinity focus distance starts? Hope TS won't mind me asking this question here.
 

ST1100

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Jun 18, 2003
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#9
And my question is:
What is the difference between hyperfocal focus and infinity focus. Where does hyperfocal focus distance stop and infinity focus distance starts?
The two terms are different concepts.

Every lens can focus in one and exactly one plane. A lens can focus 1m away, 5m away, 30m away, etc.

There is a zone of 'acceptable blur' in front of that focus plane (front DOF), and a zone of 'acceptable blur' behind that plane (rear DOF). These two zone add up to become the 'depth of field'. This depth of field is determined by 4 factors: subject distance, focal length, aperture and 'circle of confusion' (coc). The last term is an arbitrary (but industry-accepted) number based on how much enlargement is expected from film to print.

As the focusing plane gets further and further away, the rear dof increases, up to a point where it is so large that it is effectively 'infinity'. Hence, designated 'infinity focus'. Two notes: 1) point of 'infinity focus' can change when a different coc is chosen and 2) some lenses can focus 'beyond infinity' based on the - since the distance scale on the lens is really tied to an arbitrary coc.

Hyperfocal distance refers to that minimum distance where the rear DOF is considered large enough to be 'infinite' (again, computer based on the 4 factors above). The lens can focus beyond that point, although on some lenses they might be marked as the same point.

An example of usage: Let's say i have a scene where i want as much dof as possible, and hyperfocal distance of my lens is 2.5m. i would set my lens to the hyperfocal distance. The plane of focus for my lens would be 2.5m, and subjects at exactly 2.5m away would be the sharpest elements in the picture.

However, let's say the main focus of the picture is a flower 4m away. i would then choose to focus the lens at 4m instead of 2.5m. If the lens markings say 'infinity' is at 2.5m, i would need to focus 'beyond infinity'. The main subject would be the sharpest element in the picture, but i would sacrifice some front dof.
 

Andy Ang

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Jan 10, 2006
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#10
Faints. That's why I say just use the chart. That's the theoretical Hyperfocal Distance for the lens. But it just don't work so nicely on DX Lens anyway. (Because of the crops).
 

Jun 1, 2004
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#11
I thought the sensor size difference was compensated by changing the CoC value from 0.03 to 0.02 when calculating the hyperfocal distance? The focal length just remain the same.
 

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