Newbi question about DX

[RasikaJ]

mm..

been pondering over this for while now.. so here it it..

i know that the actual focal length on a DX camera should be calculated by multiplying the Full frame focal length by the crop factor. 50mm FF X 1.5 = 75 mm

but, is this the same for DX lenses or has that calculation been done already and the "real" focal length been labeled on a DX lens.. for instance is the focal length on a 18-55mm DX is actually 18-55 or 22.5 - 82.5mm ??

regards

Rasika

[serametin]

Originally Posted by RasikaJ mm.. been pondering over this for while now.. so here it it.. i know that the actual focal length on a DX camera should be calculated by multiplying the Full frame focal length by the crop factor. 50mm FF X 1.5 = 75 mm but, is this the same for DX lenses or has that calculation been done already and the "real" focal length been labeled on a DX lens.. for instance is the focal length on a 18-55mm DX is actually 18-55 or 22.5 - 82.5mm ?? regards Rasika

it is 22.5-82.5mm

[geraldkhoo]

Originally Posted by RasikaJ mm.. been pondering over this for while now.. so here it it.. i know that the actual focal length on a DX camera should be calculated by multiplying the Full frame focal length by the crop factor. 50mm FF X 1.5 = 75 mm but, is this the same for DX lenses or has that calculation been done already and the "real" focal length been labeled on a DX lens.. for instance is the focal length on a 18-55mm DX is actually 18-55 or 22.5 - 82.5mm ?? regards Rasika

The calculations have not been done yet for DX lenses... so you still need to multiply 1.5 to the DX lens range, so a 18-55mm is 27-82.5mm on a DX sensor.

[prolensdreamer]

no need to pay so much attention on technical term, just continue shooting................and enjoy the world of photography.

[diediealsomustdive]

Focal length marked are true focal lengths, go and look at the specs for a compact, and you will see very small numbers. Like the new Nikon COOLPIX S630 has a 7x Zoom lens with f=6.6 - 46.2mm / F3.5 - 5.3. In terms of field of view (FOV), it is probably equivalent to 35mm (or full frame - FF, FX in Nikon-land) 28-200mm or something like that.

FOV is what you see through the view finder, and users of SLR are familiar with the FX FOV, so the idea stuck on.

So if you buy an AFS DX 18-55mm, the lens focal length marked are true focal length based on physics of optics. The FOV is 18x1.5-55x1.5mm ~ 27-85mm FOV, or if you were using film or FX cameras, the view you see is similar to an old 28-85mm lens.

But due to the change in the sensor size, the depth-of-field (DOF) will be different (for the case of compact - very different). DOF will depend on the true focal length, not FOV.

Hope this helps.

[tkbonz]

Just to confirm something:

The AF-D 50mm, when used on a DX camera, does it have the same perspective as used on a FX camera, just that the field of view is different (because the sensor is cropped on a DX)?

If i use a DX only 35mm compared with a FX 50mm ON A DX CAMERA The FOV is the same, but is the perspective different? (Cause different lens physics so should have different perspective)

THanks! Hope I'm clear enough.

[karnage]

Yes, you'll have the same perspective but it'll be a cropped perspective. On the other hand, your image will be slightly different if u use a 35mm on a DX sensor and a 50mm on an FX sensor.

[ortega]

[diediealsomustdive]

Originally Posted by tkbonz Just to confirm something: The AF-D 50mm, when used on a DX camera, does it have the same perspective as used on a FX camera, just that the field of view is different (because the sensor is cropped on a DX)? If i use a DX only 35mm compared with a FX 50mm ON A DX CAMERA The FOV is the same, but is the perspective different? (Cause different lens physics so should have different perspective) THanks! Hope I'm clear enough.

Take the AFD 50mm lens and take a shot with FX camera, then the resulting picture, crop to DX - that's the perspective you'll get on a DX camera.

So a DX 35mm shot has the same perspective as a FX 35mm shot cropped to DX size. So the perspective will be different from a FX 50mm shot on FX camera, although the FOV is the same.

The same with DOF. The 35mm DX shot will have 35mm DOF and 35mm perspective, but 50mm FOV, roughly.

Post comments - bravo Ortega. But I got no FX camera. Maybe you can illustrate what i said in words? Post-post: exactly what giantcopy did.

[prolensdreamer]

picture tells a thousand word,

[giantcanopy]

Here is the 24mm prime lens on a DX sensor



And here is the same 24mm lens on full frame sensor



And a rough idea of this crop factor thing, the central crop being the view of the DX sensor



Ryan

[bigpond]

Originally Posted by tkbonz Just to confirm something: The AF-D 50mm, when used on a DX camera, does it have the same perspective as used on a FX camera, just that the field of view is different (because the sensor is cropped on a DX)? If i use a DX only 35mm compared with a FX 50mm ON A DX CAMERA The FOV is the same, but is the perspective different? (Cause different lens physics so should have different perspective) THanks! Hope I'm clear enough.

It is useful to bear in mind that perspective is simply a function of the distance between the camera and the subject. If your subject is very far away, everything looks "compressed" (although also very small when viewed from a distance). If your subject is too near, it looks distorted (ie, nose becomes disproportionately large). Whatever lens you use does not in any way change the perspective.

If i use a DX only 35mm compared with a FX 50mm ON A DX CAMERA The FOV is the same, but is the perspective different? (Cause different lens physics so should have different perspective)

Again, i think you are mistaken here. Different focal length can give you different image size but perspective depends only on where you stand relative to your subject. Say you are standing 6 feet away from your subject and you use a 35mm lens on a DX camera (never mind whether the lens is DX or FX). You will probably be able to take a half body picture of the person in front of you. Now, without changing position, you change your lens from 35mm to 50mm -- you will then prob take a portrait of just the head and shoulders. If you examine the 2 images, the perspective is the same -- just that the face of that person taken with the 50mm lens appears bigger (magnified) compared to the other. Even if you change to a 10mm lens (with subject in the centre) the perspective remains the same, just that you now "see more things" in the picture, and the subject appears much smaller relative to the rest of the picture.

Bear in mind that DX means cropped sensor. It has no influence on the focal length of the lens. The image formed by the lens is unchanged whether you use a cropped sensor or a full frame sensor. That's why many birding photographers like DX cameras because it gives them the extra "reach" without having to pay a lot more for a longer FL lens.

[diediealsomustdive]

Originally Posted by bigpond It is useful to bear in mind that perspective is simply a function of the distance between the camera and the subject. If your subject is very far away, everything looks "compressed" (although also very small when viewed from a distance). If your subject is too near, it looks distorted (ie, nose becomes disproportionately large). Whatever lens you use does not in any way change the perspective. If i use a DX only 35mm compared with a FX 50mm ON A DX CAMERA The FOV is the same, but is the perspective different? (Cause different lens physics so should have different perspective) Again, i think you are mistaken here. Different focal length can give you different image size but perspective depends only on where you stand relative to your subject. Say you are standing 6 feet away from your subject and you use a 35mm lens on a DX camera (never mind whether the lens is DX or FX). You will probably be able to take a half body picture of the person in front of you. Now, without changing position, you change your lens from 35mm to 50mm -- you will then prob take a portrait of just the head and shoulders. If you examine the 2 images, the perspective is the same -- just that the face of that person taken with the 50mm lens appears bigger (magnified) compared to the other. Even if you change to a 10mm lens (with subject in the centre) the perspective remains the same, just that you now "see more things" in the picture, and the subject appears much smaller relative to the rest of the picture. Bear in mind that DX means cropped sensor. It has no influence on the focal length of the lens. The image formed by the lens is unchanged whether you use a cropped sensor or a full frame sensor. That's why many birding photographers like DX cameras because it gives them the extra "reach" without having to pay a lot more for a longer FL lens.

You are right about perspective when there is no change in distance, and if there is no change in distance. Just cropping at the same distance it should show the same perspective.

If you choose to use 35mm DX to do the same FOV as 50mm, i.e. there is a change in distance, the perspective will alter. You are right. I stand corrected.

[tkbonz]

Let me try to put it this way:

Assuming the distance between the cam and the subject remains constant...

Then DX 35mm on DX cam has the same FOV and perspective as a FX 50mm on a FX cam? Cause even though its a DX 35mm, but it projects a smaller image circle which fits the DX cam's cropped sensor which result in it having the same final image's FOV and perspective as a FX 50mm on a FX cam.

If this is the case, it also means that under the above assumption, FX 50mm on DX cam has a different FOV but same perspective as FX 50mm on FX cam? Both same image circle size but due to the cropped sensor of the DX cam, it appears "zoomed" in.

Correct me if i'm wrong dudes. Thanks for the many inputs!

[Grouch23]

Another Visual example:

http://nikonasia-en.custhelp.com/cgi...p?p_faqid=4790

Hope it helps

[ortega]

another test that i did



used a 18-70mm zoom on a tripod
one shot at 18mm and the other at 70mm
then cropped both to fit above the other

if you can see the animated gif file, you will notice that the perspective is the same
this also demonstrates the DOF at different focal length

[5kywalker]

Originally Posted by ortega another test that i did used a 18-70mm zoom on a tripod one shot at 18mm and the other at 70mm then cropped both to fit above the other if you can see the animated gif file, you will notice that the perspective is the same this also demonstrates the DOF at different focal length

Just curious, am I wrong to say that the perspective might be slightly different because of distortion nearer the ends for say, wide angle lenses?

[ortega]

you can try and let us know the results

[mrbinks76]

Originally Posted by 5kywalker Just curious, am I wrong to say that the perspective might be slightly different because of distortion nearer the ends for say, wide angle lenses?

I think you are right on this point. I recently shot some products for a friend and at ~26mm I get a more exaggerated perspective. At around 62mm , shooting the same products but distance from item to camera is further , the exaggeration is less pronounced. Look at the front and rear wheels. Sorry, pics aren't scaled to same exact size.

@62mm

@ 26mm

[gooseberry]

Originally Posted by mrbinks76 I think you are right on this point. I recently shot some products for a friend and at ~26mm I get a more exaggerated perspective. At around 62mm , shooting the same products but distance from item to camera is further , the exaggeration is less pronounced. Look at the front and rear wheels. Sorry, pics aren't scaled to same exact size. @62mm @ 26mm

The perspective is different because you shot the images at a different subject to camera distance.

Perspective is purely dependent on subject to camera distance, not lens focal length.