A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,011
Re: Agreed!

philosomatographer wrote:

Great Bustard wrote:

Wow. Just wow. Everything you wrote is entirely incorrect, and I will prove it wrong with a single example that you can all do at home.

Let's take a photo of a scene with an E5 + 35-100 / 2 at 50mm f/2 1/200 ISO 3200 and 100mm f/2 1/200 ISO 3200 (note that we have used the same lens, same f-ratio, same shutter speed, same ISO, same sensor, and thus same sensor efficiency and pixel size).

We now crop the 50mm photo to the same framing as the 100mm photo and display both photos at the same size. Which photo is more noisy and why?

(Hint: while the sensors are the same size, the cropped photo uses only 1/4 the area of the sensor, and thus only 1/4 as much light makes up the cropped photo as the uncropped photo.)

I have to agree - the poster does not appear to know the first thing about digital imaging.

We have had some strong disagreements in the past regarding the performance of different lenses, but I just wanted you to know that - when it comes to image sensors, we are in the same camp.

Pleased to hear it!

I know what you were trying to achieve with your hypothetical scenario, but it is, of course, not realistic. With the smaller sensor, we are usually shooting at two stops less noisy ISO, because our lens is often two stops faster, and often better in the optical department (because of telecentricity, and the fact that it's larger in relation to the sensor, and thus better-corrected).

Actually, the 4/3 lens is not "often", or even ever, two stops faster to the FF offerings.

In that scenario, the only thing that really matters is lens quality.

This is correct.  Of course, since a 4/3 photo is enlarged 2x as much as a FF photo for the same display size, the lens must be 2x sharper at the equivalent f-ratio for a given pixel count and AA filter.

Then there is the case where you are shooting at base ISO on your 35mm sensor already - and there is no "two stops less noisy" ISO 25 setting on the small sensor. Then the small sensor can offer no match for the larger sensor, no matter what the origial poster seems to be claiming.

I think the reason they don't have ISO 25 is because the lower ISO setting would result in more noisy pixels and would adversely hurt higher ISO performance.

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