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

Started 3 months ago | Discussions thread
(unknown member) Regular Member • Posts: 217
Re: Clarification.

John Sheehy wrote:

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.

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.

The aperture or pupil is one of the most real things in photography. It is true that it does not change when you crop (or when using a TC), but the resulting image, other than the change of MP (for cropping), looks just like a photo taken with a longer lens and a higher f-ratio, in terms of DOF, diffraction, and noise.

On the other hand, adding a TC changes the focal length and that changes the ratio and thus your aperture.

The f-ratio is not the aperture. F-ratios are for completing (optional) exposure triangles, and AF systems are generally affected directly by the f-ratio used for focusing, for geometric reasons (IOW, beyond the loss of light flux on the AF sensors). Multiplying the f-ratio by 1.4 with a TC may slow or disable AF, depending on the camera, even if you double the available light, because AF isn't about the quantity of light alone, but the various angles that light is coming from the lens converge from on the sensor.

If you just crop, the diffraction does not change either ... 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.

That happens because some people assume that f-ratios are visible image parameters, making them some kind of photographic currency. They are not. They seem that way and have gotten into photographic culture because lots of people used film cameras with a single prime lens, where varying the f-ratio gave different effects at the same focus distance, but the aperture or pupil was being changed, too, and that is where the visible qualities come from.

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.

Edit: Just a bit late...

Good post.  I am guilty of confusing aperture and F-ratio which confuses the topic.

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