1.6 'crop' factor and perspective ...?

[waynelittle]

I know moving to digital makes all 35mm equiv numbers higher by a

factor of 1.6. Alot of people say things like "makes your 85mm prime a

great 136mm prime".... this is all well and good, But is the lens

still an 85mm in perspective terms?

It may not be much of an issue for portrait lens' in the 50mm and

above range, but does the perspective matter at below 50mm. ie I'm

about the purchase a 24 70mm f2.8 Sigma. This should be fine for the

top end but was just wondering about the perspective issue at the

lower end...?

[Spearhead]

Lenses don't have "perspective." Perspective is a function of the relationship between the subject and the location of the camera. All lenses have the same "perspective" at the same distance. So if you stand in the same spot with a digital camera and a film camera, both with the same lens, the perspective is the same. If you move to have the same field of view, the perspective has changed.

[digitmstr]

The lens does NOT change. What changes is the CROP of the image. The lens characteristics remain the same.<div></div>

[waynelittle]

Perhaps what i'm trying to get at is the 'distortion' given to objects with a smaller focal number. i.e a close up face shot taken with a 28mm lens,on full frame sensor,would 'distort' the natrual perspective of the face (nose looking bigger etc) Yet take it with a 50mm and this would give a more realistic repoduction of the natrual 'perspectice' the human eye see's.

However would the same 'distortion' of the face occur on a 1.6 sensor camera, even though we now call it a 50mm (almost) 'natrual perspective' lens ?

Or to achieve the same 'perspective' or 'distortion'(or lack of) should i use a real 50mm lens and stand further back ?

Hope you can see where I'm comming from or am I too 'perspectivly-warped and distorted?)

 

As I understand it the optical characteristics of a lens must stay the same, we are just seeing a smaller part of what the lens "See's". So for however much the lens bends the image (Like a fish eye in extreme case) we will still get this on 1.6 sensors but with a restricted field of view.

 

Clear as Mud ?

 

Regards Wayne.

[ozone42]

You are correct in your logic. Terminology... I'm honestly not sure myself how to describe what you're getting at. I've heard it termed 'Compression' on telephoto lenses. The idea is exactly right though.

 

An 85mm lens will always be an 85mm lens. The inherent magnification will remain the same.

 

I think the example is better for long lenses. Some fashion photographers favour very long lenses for the aspect of compression. They stand far enough away from the model so that they may have to use a bullhorn or radio to give them cues, but the effect is longer noses are compressed, the background seems bigger/closer. This effect will remain the same regardless of the crop. Say you're shooting a model and behind her is a lighthouse in the distance. Using a 100mm lens the lighthouse may seem small/insignifigant in the background, but framing her properly with a 300mm lens will "bring the lighthouse closer."

 

Again I don't know the correct terms for this, just the effect. Perspective is as good a description as any.

[kevin_krumwiede2]

Wayne, the notion that short focal length lenses introduce distortion is nothing but a myth. As Jeff said, the only thing that affects perspective is the spatial relationship between the camera and the subject.

 

I think this myth is perpetuated by photography teachers who are too lazy to explain the only slightly more complicated truth. The reason a short focal length lens seems to distort subjects is that you have to GET CLOSER to make the subject appear the same size in the frame. The distortion is caused by getting closer, not by the focal length or view angle.

[christopher_bibbs]

Kevin nailed it. People think of a focal length as having a certain perspective because they always move up on their subject so the frame is filled. You'll be as close to your subject with a 30mm lens on a 1.6 crop as you would with a 50mm on a full frame (well, pretty close to the same at least).

 

The one property of the lens that doesn't change much as you move formats is the DOF. You have more DOF on the 1.6 crop of the 30mm as compared to the 50mm on a full frame. Whether this is good or bad depends on your goal.

[markci]

Perspective is the size relationship between objects at different distances. It is a matter of simple geometry, <i>does not involve optics in any way</i>, and therefore is not a characteristic of lenses. It is <i>exclusively</i> a function of where you stand.

<p>

The reason a telephoto lens compresses perspective is because it forces you to stand back. The reason a short lens exagerrates perspective is because it forces you to get in close.

<p>

Due to the cropping factor you stand further back with the digital sensor to frame the subject the same way. Hence you will not get the same perspective distortion at the 24mm setting as you would full frame.

<p>

This question has been coming up about once every couple of weeks for several years. Every time there a couple of clueless types who post the wrong answer. Some things never change.

[lhg]

Perspective compression solely depends on the camera-subject distance. If you want to frame the exact same thing with the same lens with a 1.6x crop factor camera, you'll have to stand farther away from your subject and hence will introduce more perspective compression. If you instead take the picture from the same spot, but with a 1.6x smaller focal length, you will have the same perspective.

[waynelittle]

Kevin and Christopher, I have to disagree with you at this stage of what I Know....

the idea that all lens' give the same 'perspective' or 'distortion' (distortion is what i'm primarily concerned with) must be wrong.The idea that just moving closer or further away is what changes the 'perspective/distortion' must surley be wrong also.

Standing on a fixed spot using a wide angle lens, let's say very wide, the angle of view which the lens is capable of 'seeing' has to be bent down through the refraction of the lens in order to fit on to the Sensor (wether it be film of digital, only the size is of importance)

On a telephoto lens not so much of the view needs to be 'bent' through the refraction of the lens so objects remain less, if at all distorted.

An example would be of taking a photograph if a large square grid. Taken with a wide angle lens from a fixed spot would show 'distortion in the grid and lines would not appear straight.

Taken with a longer lens not so much of the grid would be expected to be captured by the lens. Therefore not so much 'bending' of the light from outside of the lens' straight line of axis. Result - Less disstortion but from exactly the same spot relative to the subject.

 

(something tells me Einstien might have been able to explain this, as it involves relativity and bending of light.!)

 

Still I think my original point remains that taking a photograph with a 35mm equiv wide angle lens on 1.6 frame sensor, will produce exactly the same distortion to perspective (Barrel distortion if you like) as it will on a full frame eqiuvelent. And the 'Stacking' effect of long telephotos will not be repoduced from a wider lens on a 1.6 sensor. All you will get is a smaller image (cropped) of what the full frame sensor would see !

 

What I'm essentialy trying to say (I think) is that you don't get something for nothing. You are not getting higher telephoto images on 1.6 body's (less refraction & higher magnificaction) but rather just a smaller 'cropped' image of what the lens is really capable of.

 

Regards Wayne.

[digitmstr]

I don't agree with some here: distortion, as per fisheye, for example, is a introduced by the lens. The amount of such varies with lens design, etc...

 

Distance from the front lens element also changes the image because each lens has a different optical characteristic. Such character/property does NOT change with the cropped cameras. However, since you'd have to STEP BACK to achieve the same image size on your viewfinder (compared to a full-frame camera) the lens "distortion" would also change.

 

If take a picture with my 24-70 at the widest setting, with the subject very close to the lens it WILL look like a 24, no doubt about it. See example below, taken with a 24-70L @ 24mm...<div></div>

[dogbert]

The terms perspective and distortion, as used by photographers, do not mean the same thing. Distortion is generally taken to mean the apparent bending of straight lines. For example, a fisheye lens deliberately creates distortion. Other lenses depending on how well they are designed might introduce unintended and more subtle curvature of straight lines. Barrel (outwards) or pincushion (inwards) are two terms you might hear to describe the qualities of such lenses.

 

What the others have said about perspective is correct. It is a funtion of the relative distance of the camera and the objects you are photographing and has nothing to do with lenses. Wide anlges are often used to emphasise the foreground relative to the background, most probably because their greater depth of field allows both foreground and background obejcts to be sharp. But this perspective effect is not a function of the lens itself, but the relative placement of objects.

 

I agree many people don't understand this point and more often than not you will hear people saying it is the wide angle lenses that create the perpsective effect.

[Spearhead]

<i>If take a picture with my 24-70 at the widest setting, with the subject very close to the lens it WILL look like a 24, no doubt about it</i><p>

 

One more time, it's not the lens. I really think it would help if people took some basic classes at a local school. If you take the photo at the same distance with any other lens and crop to equivalent field of view, the perspective will be identical. So a 16mm lens will give an identical image to the example providing it is shot from the same distance and cropped (and assuming there are no optical design issues between the two lenses.) If you can focus at 50mm, you can shoot at that focal length from the same distance and then crop the one taken at 24mm to match. You will have an identical image.<p>

 

Perspective is a function of the spatial relationship between the subject and the camera. This really is Photo 101, and there isn't any reason to debate it.

[skipd]

Wayne - we went through this at extreme length on another forum. The bottom line to answers your question:

 

If you had both a full-frame (or 35mm film) camera with a 160mm lens and a Canon 20D (APS-C sensor size - 1.6 crop factor) with a 100mm lens and took a photo of the same scene from the same position, you will have virtually identical photographs. I say "virtually" only because there will likely be slight exposure, color, and resolution differences because of the body specifications. The image shape, however, should be identical.

 

Exposure settings should be identical for both cameras, assuming the same ISO setting and/or film speed.

 

Even with the same f-stop settings, apparent depth of field will be a little different when you print to the same size paper from both cameras. The reason is because you have to magnify the image size more from the 20D camera than from the full-frame camera to achieve the same size print. The apparent depth of field from the 20D will be a little less than with the full-frame.

 

A real problem that folks have is understanding that even though the lenses we use on today's digital SLR's were generally designed for 35mm cameras, we should forget that entirely. The important thing to know is that the "normal" lens for camera with an APS-C sensor is approximately 31mm - NOT 50mm. Focal lengths shorter than 31mm are wide angle and focal lengths longer than 31mm are telephoto. You need to isolate your thinking as much from 35mm camera numbers as you would moving to a 4x5 view camera.

[ozone42]

I think we're missing something here. Here's a good explanation of the effect I was going for.

<br><br>

<a href="http://www.hash.com/users/jsherwood/tutes/focal/focal.html">Focal length 'compression'</a>

<br><br>

Granted, those aren't real photographs, but it explains the point very thoroughly. I can frame my subject with any focal length almost identically, but the rest of the image could be substantially different depending on focal length. This is not depth of field. This is ZOOM/Magnification/Compression. I can't take a crop from these images and have identical results.

<br><br>

Forgive me again for not knowing the proper terms.

[will_perlis]

What Geoff and Jeff said.

 

If you can't prove it to yourself with your own gear you can find a series of shots showing the independence of perspective from focal length in Canon's Lens Work III book.

 

http://www.amazon.com/exec/obidos/tg/detail/-/B0000AM6UV/103-1446100-9278264?v=glance

[ozone42]

Also to clarify. The lens does not change at all.

 

But I'm also saying that a 200mm lens on a 1.6 crop factor does NOT equal a 320mm lens on a full frame camera.

[christopher_bibbs]

Alex,

 

In the example you cited, the virtual camera was moved between each shot. If you go back read Jeff and Skip's messages you'll see they specified you would need to keep the camera in the same spot.

[ozone42]

Yep I misread someone else's post and skimmed to much. I agree, if you don't move, it's exactly the same.

 

The crop factor could best be explained on the positive side as bonus enlarging potential.

[awindsor]

Alex we are not missing anything here. What is changing in those shots is the distance from the camera to the subject. In order to maintain the same subject size (the front alien) as the focal length decreases the camera has to move closer to the subject. It is changing the subject to camera distance that changes the perspective.

 

To answer Wayne's question; the "perspective" of a 85mm lens on a 1.6x crop factor camera is the same as the "perspective" of a 136mm on a 35mm camera. Perspective is dependent on the subject to camera distance only. The subject to camera distance only depends on the focal length because we try to "fill the frame". Filling the frame really only depends on the field of view. We get that the perspective of the lens depends only on the field of view. Thus the "perspective" of a 85mm lens on a 1.6x crop factor camera is the same as the "perspective" of a 136mm on a 35mm camera. Why ? To fill the frame on the crop factor body with the 85mm lens doesn't require us to get as close as it would on a 35mm camera (obviously since the frame is smaller on the digital). How far away do we have to stand ? The same distance as it would take for us to fill the frame on the 35mm camera if we had a 136mm lens.

[primoz]

I hate to tell this, but a while ago friend of mine make me change my mind about this. Until then I also said 130mm lens on full frame is not same as 100mm lens on my 1d. Truth is that you actually get exactly same photo with above combination. It's exactly as Skip wrote. Friend of mine made this proof a bit longer with more physics theory and calculations, with some photo samples etc. but end result is exactly same as Skip wrote.<br>

Lens is still 100mm lens and it's not worth $1000 more because of this, but photos are exactly same as they would be with 130mm lens on full frame camera. Besides you can do this pretty easy yourself and you will see results. Take photo at 100mm, take photo at 200mm and crop 100mm photo so it fits just center part. Then compare photos and you will see there's nothing like perspektive change, there's nothing like perspective compression etc. It will be same.

[christopher_bibbs]

The more I think about Jeff's advice, to take a basic photo class, the more I think this really helps. Not just experience using your camera, but getting the chance to use medium and large format camera. The more you switch formats, the easier it is to deal with the crop of the dSLR.

[awindsor]

Primoz Jeroncic,

 

There is a change in depth of field.

[skipd]

Just wanted to throw some other stuff out that I learned MANY years ago....

 

Let's say, for example, that you're shooting a photo of a family group with mountains in the far distance. You see how the composition looks in the viewfinder. You decide that you want to make the mountains in the distance look twice as big behind your family group, while keeping the family group the same size in the viewfinder. The way you do that is MOVE BACK to twice the current distance from the family group and use a lens of twice the focal length that you had for the initial setup.

 

The opposite is true, too. You can get closer to the foreground subject and use a wider lens to make the distant background objects get smaller.

[waynelittle]

I think as I said in one of my early posts, the question I was pondering is only realy more apparent, in theory, at the low ends of focal lengths.

I see all af the above arguments with regard to telephoto lens' as acceptable, mainly because the light entering the lens will stay pretty much straight.

I think the term perspective has gotten in the way of my original question somewhat. Yes I fully accept the relationship of perspective is dictated by relative positioning of view point and subject.

But what I was originaly questioning was that distortion of a subjects features (ie nose etc)

So if a portrait was taken with for example a 24mm lens from 1 m the subjects nose would look distorted.

If however it was taken with a 50mm lens at 1 m it would look less distorted.

The perspective would not have changed, only the amount of field of view (or left to right light. In other words Data) gathered by the lens would have changed by using a different focal length.the lens would not be gathering more 'left and right' light so wouldn't need to distort it more in order for it to fit on th sensor.

The examples you state above to try it yourself should i feel be tried with extreme low focal lengths, as the higher you go the effect will of course become less visible to the human eye. That is why you do not see any difference (I remind you of the Large grid theory i stated above with a fish eye)

 

Perhaps the effects even at low'ish numbers will be negligable, so my original question will not be a worry to my buying of the 24-70 f2.8 for portraits.

I still feel though that, in theory anyway, a lens with a given focal length will always be a lens of a given focal length, and produce the same distortion regardless of the size of the sensor recording the data passing through the lens.As this will always be constant, unless the focal length if changed.