My take on the RF 800mm f/11 DO IS STM for birds

Started 5 months ago | Discussions thread
(unknown member) Regular Member • Posts: 217
Reduce light hitting sensor.

J.K.T. wrote:

MyDog Is Incharge wrote:

Crop mode, cropping, and a TC all reduce the effective aperture. There is no way to crop or use crop mode on an F8 image and still end up at F8.

When you crop, use a TC, or use crop mode, you are throwing away light. A TC throws away light and spreads the remaining light across the full sensor. Crop modes and cropping, crops away light and uses a smaller portion of the sensor.

I'm not convinced about that.

The reduce the light used. They either reduce the light hitting the sensor, or throw away light that hit the unused part of the sensor.

If it was not true then M43 would be all the rage.

Whether you crop or not, your front element size is the same (d). When you crop, your focal length is the same as before (f) and the ratio f/d is not changed. Thus your aperture does not change. On the other hand, adding a TC changes the focal length and that changes the ratio and thus your aperture.

It basic equivalence. Put a 50mm F1,8 lens on M43 and it does not physically change. It gather the same amount of light. But 3/4ths of light that would hit a FF sensor lands outside the M43 sensor and is thrown away. so you get an image like taken with a 100mm F3.6 lens. EXACTLY the same if you do a 2x crop of a FF image. And the same thing a TC does. It spread light outside the sensor area and so less light hits the sensor.

If you just crop, the diffraction does not change either

Oh yes it does. Crop out the middle 10% of any image and make it the same size as the original. Notice how unsharp it looks and all the noise, etc.? By you logic I could take any FF image and display it at 10% on my monitor and declare it has more MORE DR and LESS diffraction!

... relative to your pixel size. Relative to to the entire image it does, but calling that effective aperture is a bit of reach in my opinion. And if you use crop body with same number of pixels as the original, your diffraction is now larger relative to your pixels too. However, your description of the effective aperture seems a bit odd in that case as well.

Pixel size has nothing to do with diffraction. A 12 MP FF sensor Won't suffer from less diffraction than a 36MP sensor. And upscaling a 24MP image with diffraction won't remove diffraction.

Edit: Just a bit late...

Real quick.  I am sure you agree that if you take a photo with a 50mm F1.8 lens and crop 3/4ths of it out leaving a "2x crop", the resulting image will look exactly like a photo taken with a 100mm F3.6 lens.  That includes DoF, noise and DR.  Right?  Basic equivalence.

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