Wide aperture - Pictures out of focus

Started 2 months ago | Discussions thread
Truthiness
Regular MemberPosts: 122
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ISO is not relevant
In reply to tedolf, 2 months ago

tedolf wrote:

Truthiness wrote:

tedolf wrote:

Truthiness wrote:

tedolf wrote:

this is one of the problems with larger sensored cameras. Wide open the DOF is just too shallow to do portrait work.

How can an option be a problem? No one is forcing to shoot wide open, but isn't it good to have a such option?

If you need to shoot wide open (need faster shutter speed, need to shoot at base ISO) it is a useless option.

But that is a condition which is outside the performance envelope of the smaller sensor camera.

An Olympus OM-D EM-5 can't shoot wide open at base ISO?

You want the FF to shoot an image the OM-D can not shoot, both regading DOF and image quality. Why can the FF shooter not be satisfied to just shoot an identical image to OM-D?

The big sensor can still do what the small can.

It can't.

It can trade DOF to more light. It can also shoot at the same DOF and collect the same light.

It can't shoot a tight head and shoulders shot with an 85mm lens wide open at 10' because the DOF is less than 3".

Again you insist "wide open" for some irrational reason.

Do you not understand, that stopping down the FF two stops give both the same DOF and image quality you would get from a m43?

A 4/3 sensor camera can (45mm lens, f/1.8 focused @ ten feet DOF = 10 inches.

Look at a DOF table.

Why can I not shoot the image on FF so that it's got the same signal-to-noise ratio (and almost certainly better optical quality due to stopping down), but am forced to get a higher SNR? Why can't you accept that I can also shoot the same image with FF. Shooting wide open creates a different image, shooting two stops closed the same image.

Please answer this question:

What is different in the image quality if I shoot on FF with 1/200, f/4 and ISO 400 and if I shoot on m43 with 1/200, f/2 and ISO 200?

(disregard image ratio, QE etc.)

And in the case you want to take advantage of the extra full frame has, you have it.

If you stop down FF, you'll get the same image quality (for the same shutter speed)

????? how are you going to get the same image quality if you need 1/125 sec. shutter speed to freeze the bride walking down the isle but you want to use base ISO for a variety of reasons?

Why you insist base ISO on both systems?

If you use a FF DSLR you are going to be shooting at 1/30 sec. to get the same DOF as on the 4/3 sensor.

No, I'll be shooting 1/125 and if I feel like it, I'll up the ISO 2 stops or just push in Lightroom.

and DOF you get from a smaller format, but if you need more speed, you can open the aperture further, to terriritory the smaller formats can not.

And once you open up the aperture your DOF shrinks and parts of your shot are going to be OOF.

But that is only an option. I don't have to do that. An FF can achieve exactly what m43 and also both better SNR and more shallow DOF (the two go hand in hand if the shutter speed remains the same).

Do you realize that f/2 wide open m43 or APS-C is not the same as f/2 wide open on full frame?

That is exactly my point! DOF is greater on the 4/3 sensor at the same aperture/shutter speed combination.

But your point fails to understand that I can stop the FF down two stops and it still matches the image quality with the same shutter speed.

f/4 on FF gets the same DOF and with the same shutter speed

No, not the same shutter speed. You would have to change your ISO on the FF camera.

So?

the same image quality a m43 gets on f/2.

It is not the same image quality if you use the same aperture, same ISO and slower shutter speed if the subject is moving!

ISO has either little or no effect on image quality. Over most of the image, outside of the deepest shadows it's quite irrelevant.

Do you understand why images are noisy? It's because light itself is noisy. Photon shot noise is poisson distributed and the standard deviation of the noise is sqrt(signal), thus if a pixel collect 10.000 photons, the noise is 100 electrons.

The ISO may (or may not) change the readnoise a bit, but since it's typically about 5 electrons or less, it's quite irrelevant (especially since the noises are added in quadrature), so in this case the 10.000 electron signal would have photon shot noise of 100, readout noise of 5, thus total noise of sqrt(100^2+5^2) = 100,12492 electrons. It isn't until the signal level of a pixel is down to 10 or so electrons when the readout noise becomes significant, thus it's not awfully relevant.

You mentioned Olympus OM-D EM-5. It actually has 6.5e read noise, which is a bit on the high side. For example Nikon D800 has just 2.7 electron read noise at base ISO, so upping the ISO would certainly not be needed.

Is this too technical? Do you understand where this is going? What do you think of this so far?

But for the cases you need more speed, you still have two stops of space with the FF.

Not unless you change your ISO by two stops.

Any why is that relevant?

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