1.6 Crop question that has been bugging me for ages.

I know I've been here a while and I should know better, but I keep thinking about this and need a little closure.

I'm of the opinion that, if you took a picture with a 17mm lens on a full-frame camera and cropped it by 1.6 to leave the centre of the image intact and then blew it up by x1.6, you would end up with the same image (including the opctical effect you get with wide angle lenses) as you would get by taking the same picture with a 17mm lens on an APS-C sized camera.

Surely this is correct because, essentially, you're taking the same picture but just getting a smaller portion of it ?

I know that you'd get less barrel distortion on the cropped image because it's on the outside edges, but you would still get the effect that the objects closest to you would appear larger than they are.

Please help !!

Gary.

I don't think so, because what you lost at the crop stage with 1.6 crop you can not get back by enlarging, if you did not capture the visual information at the extremes of the lens it wont be there to enlarge. I could be wrong but that's how I understand it, I will watch this thread for a better explanation which I am sure will be along shortly.

I would say you're right!

Be aware that this topic usually opens a can of worms though... see the "sticky of shame"

https://photography-on-the.net/forum/showthre​ad.php?t=45388

First, you wouldn't crop by 1.6, you would crop to the reciprocal of 1.6, or roughly .60.

The second part of your premise is true, but a little confusing at first reading. Maybe a slightly better way of saying it would be, if you take a picture with a 17 mm lens on a full-frame camera, and crop it to 60% of its original size, the field of view will be the same as with a 17 mm lens on an APS-C sized camera.

The 1.6 number is the effective increase in focal length achieved by narrowing the field of view. The 17mm lens in your example becomes roughly 27mm.

As Blue Tit says, any image that falls outside the sensor cannot be recovered.

Gary, Tom is on the right track but has the numbers off just a little bit too.

If you look at the area (in mm squared) of the full-frame sensor (24mmx36mm = 864) and the area of the APS-C sensor in a 20D (15mmx22.5mm = 337.5) and then do a little division you would find that you'd need to crop the full-frame image to 39% of its original area.

You could also define it as cropping the full-frame image to 62% of its linear dimensions. I arrive at that by dividing the 22.5mm long dimension of the APS-C frame by the 36mm of the full-frame.

Your premise of cropping an image from a full-frame camera to the dimensions of the image produced with the same lens on the sensor in an APS-C camera and then blowing each up to the same sized print is correct. You should get two copies the same image, assuming you cropped out of the center of the full-frame image. You have to make sure, of course, that both photos were taken from the same position relative to the subject.

There will be some slight differences due to the resolution of the sensor. The image from the full-frame camera will have 39% of the total pixels as compared to the camera's total pixel count. Take the 5D vs 20D for example. A cropped image from a 5D (advertised at 12.8 megapixels) - replicating the image from a 20D - would have approximately 4.88 million pixels while the image of the 20D (advertised at 8.25 megapixels) will have approximately 8.25 million pixels.

Thanks for the info guys, but that wasn't what I was trying to gain some understanding of.

I was trying to figure out if you would get the same OPTICAL effect from a wide-angle lens on a APS-C camera, as you would from a full-frame camera (albiet a 60% or 39% portion of it, depending on who you talk to ).

I kind of understand the effect cropping has on the AREA of the image, but I'm not that concerned about it.

I understand that if I use a 35mm lens on a my 10D, I'd get almost the same image COVERAGE as I would using a 50mm on a 5D/1Ds. But I think I'd still get the wide-angle effect of the 35mm lens i.e on a portrait, big nose/ forehead etc.

Can you see what I'm getting at ?

I hope so, 'cos I'm having a brain-fart.

Gary.

I'll take a chance on this one too, seeing that I was not "wrong" the last time. I would say yes, but the distorsion would not be as extreme at the centre of the lens where your crop is as at the edges.

quickben wrote:
I was trying to figure out if you would get the same OPTICAL effect from a wide-angle lens on a APS-C camera, as you would from a full-frame camera (albiet a 60% or 39% portion of it, depending on who you talk to ).

I may have buried the answer to this in my last post, but I did say that you would have two copies of the same image. By that, I meant that the two images would be virtually identical. The only exception would be the resolution issues that I mentioned.

Any distortion caused by the lens that falls into the APS-C sensor area would fall into the central 39% (of area) space on a full-frame image and would be identical to the distortion in the image from the APS-C sensor. Any distortion in the area outside that central 39% space of the full-frame image would be cut off in the crop, of course.

The distortion you are referring to here is perspective distortion. That is controlled purely by the distance between the camera and the subject. Lens distortion (i.e. barrel and/or pincushion distortion) come into play with some lenses but would be identical with the same lens on two different bodies and the images cropped to the same portion of the lens' projected image.

Within limits (avoiding the ultra-wide-angle lenses with their distortions), if you use a 50mm lens on a full-frame and a 31mm lens on your 10D (or any other pair of lenses with the 1.6 "crop factor" difference) and shoot the photo from the same position relative to the subject, you will again have two identical photographs (other than resolution).

Give me a break! It was like 2:30am when I wrote that.

Actually, when I said .6, I was referring to the horizontal and vertical axes, and not the area. I could have been clearer, I suppose, but essentially what I said is correct.

It's now four hours later, and I really can't improve on Skip's answer.

belmondo wrote:
Give me a break! It was like 2:30am when I wrote that.

Tom -

Your on the right track, but taking too many steps.

A 17mm lens on a full frame, cropped down to 60% of original size will be the same as the shot taken on a (1.6 factor) digital camera. The picture generated by the inner 60% of the lens is the same on both.

The only thing different, (but I don't think that is part of your question) would be the resolution. A 5D cropped by 40% give the same picture as a 20D, but the pixels are less. (rougly 12meg * .6 = 7.2 vs over 8meg for the 20D.)

belmondo wrote:
Give me a break! It was like 2:30am when I wrote that.

is there now a time-bound cogency crop factor we need to allow for?

I always get tripped up explaining this 1.6 topic to friends and coworkers. Skip and Tom's explanations are going to help considerably

Regards

The Moderator that never sleeps!

belmondo wrote:
Give me a break! It was like 2:30am when I wrote that.

Actually, when I said .6, I was referring to the horizontal and vertical axes, and not the area. I could have been clearer, I suppose, but essentially what I said is correct.

It's now four hours later, and I really can't improve on Skip's answer.

Not quite, corrected as below.

If you took a picture with a 17mm lens on a full-frame camera and cropped it by 1/1.6X to leave the centre of the image intact and then blew it up by x1.6, you would end up with the same image (including the opctical effect you get with wide angle lenses) as you would get by taking the same picture with a 17/1.6mm (~10mm) lens on an APS-C sized camera.

Sorry Lester, your correction is not correct!

16mm lens on full frame camera = 10mm lens on APS-C camera.

16mm lens on full frame camera suitably cropped = 16mm lens on APS-C camera. (Putting aside the resolution differences)