A Question to do with Sensors

[ameerat42]

Recently this Q came up:

If you have two cameras, one with an APS-C size sensor, and the other with, say, a full-frame sensor, AND,
the pixel density (pixels-per-mm or inch) is the same for both sensors (ie: individual pixels are the same size), then
is there any advantage in having the larger sensor?

It is not a trick question. Just remember that the important thing is the equal pixel densities of the sensors.
Let's state that EVERYTHING else about the cameras is the same...

[John King]

The answer is: Maybe ...



Of course there is, in some circumstances; in other circumstances, not much.

It also depends not only on the sensor but the lens that is used, and whether it has been designed for the sensor in use, or not. That is, just because one can use the same lens on both cameras (say a D5 and a D500), with the lens being designed for the larger sensor, it will not necessarily perform as well on the smaller sensor. An examination of lens results at (say) www.slrgear.com of such lenses will often reveal a pretty marked disparity between the performance in front of a large sensor versus a crop sensor.

The magnification required to display/print an image at a given final size must also be taken into account. For example, in order to be able to print a four thirds/micro four thirds image at the same resolution as (say) a 135 format image, the FTs/mFTs lens must have double the resolving power. Most do achieve this, which is why I can print my images at A2 size with resolution that is all but indistinguishable from a 135 format image. There will be other differences, of course.

My basic advice? Take the best images you can with whatever gear you have. Even my relatively crappy bottom grade lenses can produce images that are very nice. Even my dear old E-1 (5 MPx) will produce images that are excellent at A2 size, if I get things right.

There's the point. One of the best BIF photographers I know uses an Olympus E-M10 on the back of Canon 400, 500, 800 mm lenses, and uses exclusively manual focus. My hat's off to him. I couldn't do what he does, even with a D5 ...

When I upgrade a camera body, I make a list of things that WILL benefit my photography that my current gear cannot do. The list is usually very short - 5-6 things, or thereabouts ... .

As I get further into my dotage, the other thing I consider is the size and weight. These, and total system cost, are of extreme importance to me now.

Just a few thoughts, FWIW.

[Cage]

Well the obvious advantage is that the FF sensor is going to capture a lot more real estate than the scaled-down sensors from a given shooting point.

zzzzsensor size diagram.jpg

[John King]

Kev, the graphic really depends on which APS sensor you are talking about. Canon, Nikon, Sigma, Sony, Fuji, Pentax (IIRC), among others, all use different APS sensor sizes, or at least some of them do ... So methinks that the graphic might be a bit this way or that.

[ameerat42]

No, I agree with the graphic. It doesn't really matter about the specific sensors or makes. The idea is just
that they differ in size while having the same pixel density.

To illustrate it further, I have superimposed a grid on Kev's graphic. Consider the small squares to be the "pixels".
All that changes is the sensor size.

Illustration...
grids.jpg

[tandeejay]

Given that the pixel density is the same for each sensor, I would have thought the performance of the lens would be about the same (apart from a lens designed for a apcs sensor will not have a large enough image circle for the full frame)

A smaller sensor with a higher pixel density would have an impact on the lens performance as the lens would need to be able to resolve to a smaller point, but if the pixel density is the same, the same lens on the FF or APCS should resolve the same over the same pixel density

So in this case, having the larger sensor would have the advantage of higher mega pixel count.

Of course, I could be wrong, as I'm no scientist and definitely not a camera expert, but this explanation is "logical" to my way of thinking.

[ameerat42]

...but if the pixel density is the same, the same lens on the FF or APCS should resolve the same over the same pixel density

So in this case, having the larger sensor would have the advantage of higher mega pixel count.

Of course, I could be wrong...

Basically correct, but I'm just wondering if "more megapixels" is necessarily an advantage. "Wondering", mind. Not saying it is or isn't.

[tandeejay]

Basically correct, but I'm just wondering if "more megapixels" is necessarily an advantage. "Wondering", mind. Not saying it is or isn't.

ah ha, now that would come down to the question of "what do you want to do with the image?"

well, if you have the same lens, your going to need to crop on the FF sensor to get the same view angle as the apcs sensor, and you'll end up with the same number of mega pixels anyway... That is, the crop sensor is doing just that... cropping the image, so if you really do need the wider angle, then FF is better... but with the crop sensor, you could take several images and then stitch them together to get the same effect as the FF sensor...

[mpb]

It is my understanding the the DoF will also be shallower on the full frame compared to the cropped sensor.

[Cage]

I think Pixel Density is the key here.

Obviously for a cropped sensor to capture the same size image as a full frame sensor, you would have less pixels spread over the same area, and in theory, all other factors being equal, the cropped sensor would/should capture less detail.

[tandeejay]

I think Pixel Density is the key here.

Obviously for a cropped sensor to capture the same size image as a full frame sensor, you would have less pixels spread over the same area, and in theory, all other factors being equal, the cropped sensor would/should capture less detail.

I think your wrong, in this case, the pixel density is the same, so all other factors being equal, they should capture the same detail, but the FF sensor will capture detail outside the frame of the apcs sensor. The only way the cropped sensor could capture less detail would be if you had a different Lens eg. a 50mm on the apcs sensor and a 35mm on the FF, but then you have 2 different lenses... or do you consider the 50mm on the apcs the same lens as the 35 on the FF

- - - Updated - - -

It is my understanding the the DoF will also be shallower on the full frame compared to the cropped sensor.

That would only come into effect if you had a different lens? That is, "all things being equal" and you had the same lens over the FF or the apcs, the only difference in this case is how much the image is cropped...

[mpb]

- - - Updated - - -

That would only come into effect if you had a different lens? That is, "all things being equal" and you had the same lens over the FF or the apcs, the only difference in this case is how much the image is cropped...

Yes, but if you are shooting the same scene with a FF and cropped sensor you need to change focal lengths to get the same image, or move position.
If you just shoot with "all things being equal" then all things are not equal because you get a different field of view.

[tandeejay]

Yes, but if you are shooting the same scene with a FF and cropped sensor you need to change focal lengths to get the same image, or move position.
If you just shoot with "all things being equal" then all things are not equal because you get a different field of view.

So comes back to the OP then... What does AM consider as "all things being equal"?

I took the description of "everything about the camera is the same" to mean the same lens... if you change the lens to keep the same FOV, then you have moved away from that statement?
@ameerat42; we need your interpretation here...

[ameerat42]

Yep. Somewhere in the mix upstairs I did say keeping the same lens.
Some thoughtful reasoning here, Tands.

In the OTHER discussion I saw about this (I kept the wording the same), they IMMEDIATELY jumped to a
discourse on PIXEL SIZE. I mean!--- It had been spelled out there too.

I think there are cameras in the major makes that differ by only sensor size. Can't bring any to mind.

Ta for all responses so far.

[Hamster]

No. Next!

[ameerat42]

Seemingly, well summarised, Alex

[Hamster]

Seemingly, well summarised, Alex

[John King]

Am, this is very similar to the discussions about cropping xx MPx out of the centre of a 135 format frame to achieve the same result as that obtained from a crop sensor with the same xx MPx and the same size as the cropped area from the 135 image.

Unfortunately, it only works that way in (simplistic) theoretical terms. In fact, for this to work, the 135 format lens has to have the same, or greater, resolution than that of the crop sensor lens. This is rarely, if ever, the case in reality.

One uses lenses that have much the same angle of view for a given sensor size (leave aspect ratio out of it, it just further complicates things!). So we all use the convenient shorthand of referring to a 300 mm lens on four thirds as having an effective focal length of 600mm on 135 format. This is because the AoV is roughly the same for the 300 vs 600 lens.

However, it has to be stressed that this is just a convenient shorthand!
Neither the FL nor the aperture actually changes. An f/2.8 300 lens is functionally the same on any format. So an f/2.8 300mm lens on FTs will be roughly the same as an f/2.8 600 as far as AoV and exposure are concerned. However, the DoF of the FTs 300 mm is the same as the DoF of a 300 mm on a 135 format camera - i.e. it will have greater DoF than a 135 format 600mm lens. This is almost always an advantage, but not in every single case! However, the design of the lens will change dramatically from format to format, even if the major specifications are the same or similar. Image circle is one obvious change, another is the degree of telecentricity required. Registration distance will mostly be different. Retrofocal design will be different. etc, etc.

In the end, I find this a very fruitless discussion, that is almost always an attempt to assert the superiority of one format over another.
While each format does have strengths and weaknesses, IMO the only real benefit of examining these is when one is choosing which to buy and use for one's own needs and photography. ALL modern ILC cameras will take better photos than we mostly can ... .

If I want very clean ISO 6400 and this is always a major issue for me, then I would choose a D3x or D4s over my Olympus 4/3rds cameras. Not that the E-M1 cannot do this, and remarkably well, it just will not be as clean in all circumstances as a D3x. e.g. a couple of almost OoC JPEG shots from said E-M1 at ISO 6400 (no NR applied, it is an automated PS action I wrote that I use to batch prepare OoC JPEGs for web use):




Ditto, ISO 12,800, where my E-M1 can be decidedly flaky, and PP from the RAW is all but essential, but I didn't need a tripod for this shot either - hand held at f/5 @ 1/6th from the moving pedestrian bridge over the Yarra at Southbank:



Just a few thoughts, FWIW.

[ameerat42]

Ta for your thoughts, John. (I think I owe you 1d for them )

I guess the point you make: [...if the lens fits...] is all we can draw from it.
Ie, that you get a wider scene coverage per given lens on the larger sensor - all else being equal.

[mpb]

Ie, that you get a wider scene coverage per given lens on the larger sensor - all else being equal.

The advantage of using the same FF lens on an APS-C sensor is that you crop off any of the soft corners the lens may have, maybe.