Something to ponder!

I thought I understood the debates about full frame vs crop factor cameras until I started looking at the angle of view of the two lenses below and from my understanding the sigma is a full frame lens model.

Anyway take a look through the specs below.


Sigma 24mm -70mm F2.8 IF EX DG HSM

Angle of View 84.1 – 34.3

Focal Length 24mm to 70mm


Pentax DA* 16mm to 50mm

Angle of View 83 - 31.5

Focal Length 16mm to 50mm

Focal Length Equivalent to 35mm format 24 - 75mm

My question is this, if the Pentax DA* 16mm to 50mm lens has an equivalent focal length equivalent to 35mm format 24mm - 75mm and an angle of view of 83 - 31.5

then If both images were taken at say ISO 200 at 1/125th, on an APS-C sized sensor and a full frame sensor, then the images would look the same in what was captured in the image (point of view) or am my overlooking something?

Hope the above makes sense!

The angle of view is just that regardless of format. There was a suggestion some years ago that we should start talking about, say, a 75 degree lens and that would give a better usea than focal lrngth. Never caught on.

If you are using a 16mm lens with an APS-C sensor and then wanted to get the equivalent image with a full frame sensor you'd have to use a 24mm lens.
Using that 24mm lens with the APS-C sensor you would only get approximately half the image, as if you had zoomed in.
Field of view figures can be confusing as sometimes they quote the horizontal angle and sometimes the diagonal, so make sure you are comparing like for like. Also not all APS-C sensors are the same size so you may find figures for a non Pentax sensor (eg Canon APS-C sensors are smaller than the Pentax ones giving a 1.6x crop factor instead of our 1.5x!)
Also worth considering is that the image produced by some lenses cover the full frame sensor whereas some do not, having been designed for APS-C to save size and weight.
The ISO and shutter speed used are irrelevant.

John K

Thanks Guys.

I guess the point i was trying to illustrate is, you can capture the same field of view with an APS-C camera as you can with a full frame if you know how to.

It would seam that not all of the arguments to the advantages of full frame camera bodies are so simple, yes full frame provides better control over DOF and higher ISO to noise sensitivities.

However, when people start talking about the advantage of full frame focal lengths, if you know how to convert the two formats, in terms of field of view, personally i can't see the advantage full frame has.

JAK, I was only referring to ISO and shutter in relation to creating the same image in relation to how it would look, but I had not thought about the horizontal angle and the diagonal field of view measurements.

I thought that the KEY difference is dof and perspective. To take it to the extreme, if the sensor was iso 2 million and you had no noise, then you could shoot everything with an ultra-ultra wide 5mm lense and just crop away to get the effective fov you want, but you will always get the perspective of a 5mm lens, i.e. large separation of foreground and background - which probably won't help your portraits (big nose!)

I had an Olympus OM10 in the 80s and when I took up photography again 4 years ago I remember wondering what was missing - the fact is a 50mm APS-C lens is NOT the equivalent of a 75mm full frame lens. It will give the same fov, but not the same picture.

I think !

Perspective has nothing to do with focal length. It is entirely to do with how far away from the subject we are.

The only reason people thought FF had an advantage was that wide angle lenses remained wide angle. On APS-C they crop the image. However, since then we have had ultra short focal lengths that solve that problem.

The idea that "at least my 50mm is a real 50mm" is a bit of a misunderstanding, because a 50mm lens will always be a 50mm lens. Same DOF, same perspective from any given position. Different formats put behind it just dictate how much of that view we capture, a lot of it (wide angle) if it's a larger format, less of it (telephoto) if the format is smaller.

A lens does not change its properties because of what you place behind it.

Is this true? See here: http://www.photozone.de/focal-length-and-perspective

Though reading your last sentence I get the feeling that you agree with me? I.e. a 50mm lens will always give a 50mm perspective. I think this is crucial because people talk about the "ideal " portrait lens from film days as being roughly 85mm, so assume their 50mm is pretty much the same. But it was the perspective of the 85mm which is important.?
(I'm going to have a coffee - too much thinking for a Monday morning )

What I wrote is absolutely true Tony. If you think about it, it has to be.

What happens if you have a lens and you put a sheet of 5" x 4" film behind it? What happens if you put a 35mm frame behind it in the same position? Think about that and you'll see the lens itself doesn't change at all. What does change? Once you've got the answer you'll understand.

Whoever wrote that PhotoZone article needs a good slapping, and his computer confiscating.

I know I sometimes cross swords with JR, but on this occasion he's bang on.

Don't blindly believe everything you read on the internet (including this).

I don't believe a word of what Chris has said!

The PhotoZone article is spot on. If it were anything else
a magnifying lens couldn't exist and all optical
science would have to be re-written

The article is self explanitory.

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John

I understand what you're saying, but that isn't what I'm saying, in fact is the other side of the coin! Of course the perspective doesn't change if you don't change the lens, but I'm saying that if you want the same perspective as a 85mm lens in 35mm format in APS-C, then you have to use a 85mm. Your 50mm lens in APS-C will not give you the same perspective as a 75mm lens in 35mm format. Is this correct or not? Or am I just getting more and more confused

Tony

Algy, at risk of getting my wrists slapped by JR, you're a numpty (and/or a dreadful stirrer!)

The problem with that article is that it's shockingly misleading. The reason the author got the different perspectives is that he moved between shots.

At 17mm he was close to the lamp, so it looked bigger relative to the background than at 100mm, when he would have had to move back to get the lamp to be the same size on his image.

If he had stood in the same place, taken images at different focal lengths, and then cropped out appropriate bits to compare the perspectives, he would have found that the perspective was exactly the same in each one.

Don't take my word for it, try it!!

I don't know why you bring up the subject of magnifying lenses (I assume you mean, as in magnifying glasses) - they create virtual images not real ones like in a camera.

No Tony. And certainly no Algi, you haven't looked at their images properly.

Tony, if you want the same field of view and perspective to a 85mm lens as on 35mm film, then the nearest in the Pentax range is the 55mm f/1.4 lens.

The reason this is true is that you need to stand in the same place to get the same picture. If you had an 85mm lens on your Pentax DSLR that would be no good, you'd be cutting off parts of the subject. You'd have to move back to get it all in. So you change to a lens with a shorter focal length and get the same FOV as your 35,, camera.

Consider you are standing looking at a subject and you have several cameras with you with their standard lenses on. With your Minox sub-miniature the focal length is 15mm, with your Pentax Auto 110 it's 24mm, with your K-3 it's 35mm, with your LX it's 50mm, with your Hasselblad it's 80mm.....and so on. But they are all a standard lens and all give the same perspective because you haven't moved.

If you're having difficulty with this, and it can be tricky I'm sure you're not alone, then a book on photography in general should explain it very well. Try "The Lens Book" by Hicks and Schultz.

The 85mm will be a magnified view irrespective of format.
The 50mm will be a less magnified view. I've never actually
carried out the test to see which lens sees the same as my
eye without any magnification. Quite easy to do.... just
watch how an artist views a scene by holding up a pencil.
They say it's 50mm, but I don't know for sure.

For landscapes you can simulate FF frame by using APS-C in
portrait/vertical orientation and taking 3 shots overlapping
slightly less than half of the frame and stitching.

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