[UncleVanya]

I tend to agree with what everyone is saying about the mirror lens difficulty and the relative inexperience of people using them. But there is an underlying thread in the reviews echoing what Uncle Vanya said. (that actual focus is very hit or miss.)
But 500mm f8 isn't all that fast.
if you think about it the dof of this 500mm f8 at 10metres on a flower is exactly the same as photographing that flower with a 50mm f8 at 1 metre.
And no one has difficulty doing that.
Of course camera shake etc come into account.
(Also I think atmospherics come into account with some peoples results)

As a percentage of the subject and the distance that 500mm is much harder to focus accurately.

[GUB]

As a percentage of the subject and the distance that 500mm is much harder to focus accurately.

The flower should be at the same magnification and the dof the same.
But of course keeping the camera steady is an issue.
And the concept of a mere 120mm of dof at that distance seems foreign but in reality if you rock forward and back to achieve final focus it is the same amount of rock as the 50mm.

[rvandenbrink]

Mirror lenses are mostly fixed aperture and manual focus, which really makes things tough if that’s your one telephoto and you’re used to autofocus lenses.
If you are used to manual focus, esp on longer lenses, then it’s no different on a mirror than on standard lenses - the throw on the focus ring might be different tho
Getting the same sharpness though is a lot harder - most of my mirror lens shots look like 1975, that’s just an artefact of using that lens. Which is fine, when I use mine that’s the look I’m going for. (Mine’s a Tamron 500mm 55BB)

[UncleVanya]

The flower should be at the same magnification and the dof the same.
But of course keeping the camera steady is an issue.
And the concept of a mere 120mm of dof at that distance seems foreign but in reality if you rock forward and back to achieve final focus it is the same amount of rock as the 50mm.

The amount of focus ring angle change is much lower.

[GUB]

The amount of focus ring angle change is much lower.

Maybe as a manual focus operator that is the defining thing. That I resort to moving forward and back to achieve fine focus.
Negates the focus ring thing.

[GUB]

Mirror lenses are mostly fixed aperture and manual focus, which really makes things tough if that’s your one telephoto and you’re used to autofocus lenses.
If you are used to manual focus, esp on longer lenses, then it’s no different on a mirror than on standard lenses - the throw on the focus ring might be different tho
Getting the same sharpness though is a lot harder - most of my mirror lens shots look like 1975, that’s just an artefact of using that lens. Which is fine, when I use mine that’s the look I’m going for. (Mine’s a Tamron 500mm 55BB)

When you get lucky they can do great results. Just seems to need more luck than a refracting lens.

[rpjallan]

Focussing an f/8 lens is difficult as the viewfinder is quite dark, split screen finder would show a black wedge.

Yes, I had (actually I still have it) a plain matt focussing for my MX to use with the 400-600.

[steephill]

I remember seeing old reviews of mirror lenses which commented that the stated aperture was often very optimistic, usually by at least a stop. Now the same is often true for normal tele lenses but it seemed that mirror lenses were worse in this respect.

[GUB]

I remember seeing old reviews of mirror lenses which commented that the stated aperture was often very optimistic, usually by at least a stop. Now the same is often true for normal tele lenses but it seemed that mirror lenses were worse in this respect.

That is a design factor of the central obstruction.
The f stop is a calculation of the optics and doesn't subtract the central part.
The t stop (transmission) of a mirror lens tends to be about 2/3 of a stop slower than its f stop.
But the mirror f stop will be accurate for dof calculation

[photoptimist]

Isn't the donut bokeh of the mirror lens a likely culprit?

On a refractor lens, the out-of-focus blur circle is a circle. That blur circle adds just a bit of softness to the image that can be readily fixed by a bit of sharpening in post.

On a reflector lens, the out-of-focus blur "circle" is a donut. That blur donut adds both softness and artifacts to the image that cannot be fixed by sharpening in post because sharpening intensifies the artifacts.

Notice that the DoF calculator in GUB's post uses 0.020mm as the acceptable diameter of the circle of confusion or the blur circle. That's a 4 pixel diameter circle for cameras with 5-micron pixels which is probably unnoticeable to all but persnickety pixel peepers. But a 4 pixel donut (probably with a 1-pixel ring around a 2 pixel hole) is much more noticeable.

So maybe images with a refractor are acceptably in focus with a 0.020mm circle of confusion but a reflector lens has a tighter (say 0.010mm) blur circle constraint. That gives the reflector lens 1/2 the DoF of a refractor lens even if they have the same f/stop aperture.

[GUB]

That's a 4 pixel diameter circle for cameras with 5-micron pixels which is probably unnoticeable to all but persnickety pixel peepers. But a 4 pixel donut (probably with a 1-pixel ring around a 2 pixel hole) is much more noticeable.

Not sure that I am buying that when you take into account each pixel has an array around it as you describe and all are overlaying each other cancelling variation.

Thought I would have a go at demonstrating what I am thinking in the real world. The result is far too uncontrolled to make any serious conclusion but was interesting anyway.
The problem is there is no mirror lens without a central obstruction so I decided to use a refracting lens with an obstruction.
I took my A series 50mm f2.8 (chosen because it is sharp wide open) and put a 15mm black sticky dot on it's appr.20mm front element. That calculates out at anything with a dof greater than that of a f3.3 aperture is obscured. Obviously this is a much greater obstruction than your average Mirror lens.
Remember that the outer edge of a lens has a lower optical performance so you can expect the image to be not quite as sharp as the clear lens with its sharper internal sections enhancing the image. (That comment in itself reinforces my original argument too doesn't it?)
The top half of the crop is totally about bokeh and left in there to show how well this dot has replicated a mirror lens.
And a bokeh donut is visible in that background bokeh near the bottom.
I focused on the vertical frondlet and you can see some degradation in the dot view. (I didn't change focus after applying the dot.)
The frondlets bottom left and the stem interest me as they are that "just out of focus" area that I was hoping to see a difference.
And I believe they have softened just a little more in the dot image.
But a far too inaccurate a setup to be a definitive result.
Of course this is not an exposure comparison and I have brightened to dot image to match the other.

[photoptimist]

Not sure that I am buying that when you take into account each pixel has an array around it as you describe and all are overlaying each other cancelling variation.

Thought I would have a go at demonstrating what I am thinking in the real world. The result is far too uncontrolled to make any serious conclusion but was interesting anyway.
The problem is there is no mirror lens without a central obstruction so I decided to use a refracting lens with an obstruction.
I took my A series 50mm f2.8 (chosen because it is sharp wide open) and put a 15mm black sticky dot on it's appr.20mm front element. That calculates out at anything with a dof greater than that of a f3.3 aperture is obscured. Obviously this is a much greater obstruction than your average Mirror lens.
Remember that the outer edge of a lens has a lower optical performance so you can expect the image to be not quite as sharp as the clear lens with its sharper internal sections enhancing the image. (That comment in itself reinforces my original argument too doesn't it?)
The top half of the crop is totally about bokeh and left in there to show how well this dot has replicated a mirror lens.
And a bokeh donut is visible in that background bokeh near the bottom.
I focused on the vertical frondlet and you can see some degradation in the dot view. (I didn't change focus after applying the dot.)
The frondlets bottom left and the stem interest me as they are that "just out of focus" area that I was hoping to see a difference.
And I believe they have softened just a little more in the dot image.
But a far too inaccurate a setup to be a definitive result.
Of course this is not an exposure comparison and I have brightened to dot image to match the other.

Great test!

You might try photographing a millimeter ruler at a shallow slanted angle. The dense B&W lines progressing from foreground to background provide a consistent subject pattern at each distance and provide some gauge of the DoF in millimeters or centimeter.

[GUB]

OK how about this?
I created a polka dot pattern and full screened it on my 42in tv monitor.
Took the shots from a bit of an angle to show dof.
You can not read much from comparing the basic shots.
I use edge detect in my editing sharpening process and have noticed how well it can demonstrate the sharpness of a shot and also identify the plane of focus.
So here it is to compare the two - the whiter it is the sharper it is.
I feel it reinforces my original comments.
Of course as said previously you cannot expect the dot impact to peak at a sharpness as high as the undotted image.
But the dotted image drops off distinctly quicker.