[MattGunn]

On another note, my Tokina developed fungus and had to be trashed.

Fungus is not the end for lenses, do not trash them! I have removed fungus from half a dozen lenses in the last couple of months and all of them are fine now. If it has been there too long it will damage the lens coating and so the contrast will be reduced marginally but the lense should be quite usable.

-- and the aperture of F/8 (which is a photographic aperture of about T/11 if you take the central obstruction into account) is simply not fast enough for anything but subjects in bright light.

The central obscuration is taken into account when the f number is calculated. The entrance pupil of the lens is greater than 1/8 of the focal length so the open light collecting area of the lens is the equivelent of f/8. From measurements of my 500mm f/8 Centon and some back of the envelope calculations, the effective aperture is f/8.3

[Zav]

I guess the Minolta RF Rokkor-X 250mm F/5.6 is the shortest, at least among lenses available to the general public -- there were strange mirror lenses of all focal lengths designed for scientific and military applications.

The Tamron SP 350mm F/5.6 is also one of the shortest and lightest mirror lenses available. Here are a few pictures taken in Cambodia with this lens (sorry if you have already seen them):

And if you ask me, I don't think it's suitable to remove the donut bokeh with Photoshop. Most of the time, it looks just nice! And what's the purpose of using a mirror lens if you make the pictures look as if they were taken with a not-so-sharp ordinary telezoom?

Cheers!

Abbazz

Thanks for your answer, I really love this donut sea!

[eccs19]

I just recently got a Tamron 500mm F8 lens, and had been thinking of starting a mirror club. Talk about good timing. Here's one from a couple of days ago. Somewhat cropped.

[Rense]

Tokina 500/8 mirror. I own this lens only a few weeks. I've shown this shot before in the PYP section, without receiving comments. I can see why. But I'll keep practicing....

[MattGunn]

I got a cheap collapsible rubber hood for my Vivitar, but it isn't very effective. It helps contrast a little in case where flare wasn't a big problem to being with, but helps essentially not at all in cases where flare really kills the shot. At some point I'll get a better hood, because I know from other lenses that this can make a huge difference.

BTW, I posted a blog article a few weeks ago on my Vivitar mirror lens. For the most part, my comments on getting the most out of it apply to all mirror lenses:

Marc Sabatella: The Vivitar 500/8 Mirror Lens

Great stuff, this blog was great, motivated me to dig out my mirror lens and take some photos with it again.
For the last few months I have been using it as a spotting scope with a home made T mounted erecting prism and eyepiece. It makes a good spotting scope, much smaller and lighter than a similar refracting scopes.
Anyhow, the sun came out and so did the mirror lens. A couple of photots from it bellow, no PP other than cropping and resizing. Centon MC 500mm f/8, I think another re-branding of the Samyang / Vivitar.

Based on advice here and in Mark's blog I went searching for a suitable hood on ebay last night. I found a few but got sidetracked and brought a Tamron 55BB mirror lens in need of some TLC and instead If the faults are as described thn I should be able to get it back in good working condition, in which case I have a good mirror lens at an even better price If not...

[Abbazz]

The central obscuration is taken into account when the f number is calculated. The entrance pupil of the lens is greater than 1/8 of the focal length so the open light collecting area of the lens is the equivelent of f/8. From measurements of my 500mm f/8 Centon and some back of the envelope calculations, the effective aperture is f/8.3

When I compare the effective aperture of my Tamron 350/5.6 with any other lens (based on the shutter speed needed to take a normally exposed picture), I find it to be F/8, while my Tamron 500/8 is in fact F/11.

Maybe some brands take into consideration light losses due to the central obstruction and to the less than perfect reflective surfaces of the mirrors but apparently Tamron doesn't. I would be curious to know if you get the same exposure from your Centon than from a refractive lens at the same aperture.

Cheers!

Abbazz

[Rense]

Tokina 500/8 handheld, @ ISO1600

1/60", somewhat cropped



1/125", not cropped

[MattGunn]

A quick test reveals that the Centon with its effective aperture of f/8.3 (based on dimensions only) gives about the same exposure as my Super Takumar 300mm at about f/10. The fact that the mirror lens is slower is not due to the central obscuration though, it is due to the losses from the mirror.
At normal incidense glass reflects about 4% of the light hitting it so 96% is transmitted (assuming it is good quality glass and there is no absorbtion, this is a valid assumption for most opticval glass in the visible region of the spectrum). A good antireflection coating will reduce the reflected light to less than 1% and so more than 99% is transmitted.
Aluminium which is used for the mirror coatings typically reflects about 90% of the light incident on it so at each reflecting surface you lose about 10%. In a mirror lens there are two reflecting surfaces, the primary and secondary mirrors. This means that about 19% of the light entering the lens is lost at the reflecting surface amd this is what givess the effective aperture of f/10-11
It is now possible to produce an enhanced aluminium reflecting surface with something exactly the oposite of an antireflection coating. These will reflect more than 95% of the light and so would give a significant improvement to the light transmission. This is probably not included in most mirror lenses due to cost, age etc. Reflecting surface do have a significant advantage over lenses though as they do not suffer from dispersion and so do not introduce chromatic aberations.
If, when I get my Tamron lens I find that it has fngus on a mirror surface I will get it re-coated with the enhanced coating and see if it improves things at all.

[ivoire]

Tokina 500/8 handheld, @ ISO1600

1/60", somewhat cropped



1/125", not cropped

Nice work with that tokina

[ivoire]

There are some fantastic photos here!
I quite like the donut bokeh, especially in highlights in the water on your 5th shot abbazz.
I have a Centon mirror lens which as far as I can tell is physically identical to the Vivitar morror lens I borrowed from work. I need a bit more practice with it before I have anything worth posting though. I have had pretty poor contrast in most of the shots I have taken with it and have been considereing getting a hood for it. Has anyone used a hood with a cheep mirror lens? Does it help much (assuming not shooting into the sun)?
I have also been looking at the Tamron morror lenses which appear to produce superior results to pretty much everything else. Those of you lucky enough to have a tamron mirror lens, do you have the earlier 55B mode or the newer 55BB model? Apparently there is a significant difference between the optical designs.

found this on the 55b-55bb:

Tamron SP 500mm F/8 Mirror Model 55BB: This later version SP 500mm F/8, introduced in 1983, is an optically revised version of the original model 55B lens first introduced in 1979. The major optical design change is the switch to a constant thickness meniscus main mirror in order to reduce close focusing spherical aberration. The change in the main mirror design also necessitated changes for the two smallest lenses located within the middle of the optical design. This model 55BB also discards the earlier model's detachable tripod mount and the set of four filters for B&W photography. The later model 55BB is also readily distinguished from the original model by its diamond knurled rubber focus grip and the lack of the locking screw used on the earlier model 55B's built-in rotating tripod mount ring. Thus this lens does not feature any built-in tripod mount whereas the original lens featured a detachable tripod mount.

We haven't had the opportunity to compare both optical versions since the fact that the optical design had changed slightly completely escaped our attention until now. Nevertheless this is now a high priority on our list in order to answer two questions: Which lens is sharper across the field for distant subjects? Which lens is sharper for tele-macro work? It may turn out that the earlier version is slightly superior for long distance photography while the later version likely is somewhat superior for tele-macro work. Why do we say this without having compared these two lens versions? Because at least in the early 1980s Tamron, unlike many other competitors, was NOT known for introducing revised lens designs that offered inferior overall performance. This is because Tamron was still trying to increase consumer acceptance and market share by offering improved optics.


Lens Specifications:
Lens Model 55BB
Focal Length 500mm
Aperture Range f/8
Angle of View 5°
Optical Construction
(Groups / Elements) 4 / 7
Min. Focus from Film Plane 66.9" (1.7m)
Macro Mag. Ratio [w/2X Converter] 1:3 [1:1.5]
Filter Size 30.5mm (rear), 82mm (front)
Diameter 3.3" (84mm)
Length at
[w/Nikon mount] 3.4" (87mm)
[3.6" (91.5mm)]
Weight 21.0 oz. (595g)
Lens Hood Screw-in type, retractable.
Accessory Accepts SP 2X tele-converter #01F. Supplied with lens hood, case.


Tamron SP 500mm F/8 Mirror Model 55B: When introduced in 1979, this lens was the smallest and lightest 500mm mirror telephoto ever produced by any manufacturer. Remember the days of using Vivitar's heavy solid Catadioptric telephoto lenses or other manufacturer's far less compact mirror telephotos? This lens is significant because it changed the thinking of optical engineers throughout the industry since this lens uses Mangin mirrors. Mangin mirrors are simply lenses with one side of the lens being either aluminized or, in the case of Tamron's mirror lenses, silvered for peak reflectivity. Light passes and refracts through the unsilvered front side of the lens element, reflects off the aluminized or silvered back surface, and then the light again passes and refracts through the front side of the lens element. The whole point of using Mangin mirrors within a catadioptric lens is to reduce weight and size by getting away from the heavy solid catadioptric lens designs and earlier conventional mirror lens designs which were nowhere as compact.

Many other manufacturers, subsequent to the introduction of this lens, followed suit by designing cat lenses with very similar optical designs. For example, a year or two later Tokina introduced their very similar 500mm F/8 which had somewhat worse overall performance. For a while the Tamron and Tokina lenses were the two best 500mm F/8 mirror lenses on the market, but that changed after the mid 1980s as some other manufacturers managed to somewhat improve these landmark optical designs.

Overall, this lens produces fairly sharp images which are slightly soft in the corners. It is lightweight yet features solid construction, accepts rear 30.5mm thread-in filters, and produces acceptable performance when used with the Tamron SP 2x teleconverter. This lens also continuously focuses down to 1:3 macro. Macro performance is good but with softness in the corners due to off-axis coma. Light transmission is very close to F/8 and light fall-off is a mere 0.75 stop. Color rendition is very good. Out-of-focus doughnuts have a smaller than average "hole" since this lens has a relatively small central obstruction compared to most other 500mm cat lenses. This is a very good mirror lens with performance which was only somewhat exceeded by a few of the OEM lens manufacturers.

Lens Specifications:
Lens Model 55B
Focal Length 500mm
Aperture Range f/8
Angle of View 5°
Optical Construction
(Groups / Elements) 4 / 7
Min. Focus from Film Plane 66.9" (1.7m)
Macro Mag. Ratio [w/2X Converter] 1:3 [1:1.5]
Filter Size 30.5mm (rear), 82mm (front)
Diameter 3.3" (84mm)
Length at
[w/Nikon mount] 3.4" (87mm)
[3.6" (91.5mm)]
Weight 20.3 oz. (575g)
Lens Hood Screw-in type, retractable.
Accessory Accepts SP 2X tele-converter #01F. Supplied with tripod mount ring, lens hood, case, and 5-piece filter set.
SP 500 F/8.0 Model 55B
PERFORMANCE
Modern Photo's
Standard as Tested
Focal length: ±5%
(475-525mm) 508mm
Aperture: ±5%
(f/7.6-8.4) f/8.0
Distortion:
(±2.5%) 1.2% (pincush.)
Light falloff: at f/8
±1 stop from theoretical limit 0.75 stop
SP 500 F/8.0 Model 55B
F/no. RESOLUTION
at 1:40 magnification CONTRAST
(30 lines/mm)
Center
Lines/mm Corner
Lines/mm Center % Corner %
8 48 36 44 38
F/no. RESOLUTION
at 1:4 magnification
8 40 30
Minolta 500mm F/8
PERFORMANCE
Modern Photo's
Standard as Tested
Focal length: ±5%
(475-525mm) 503mm
Aperture: ±5%
(f/7.6-8.4) f/8.3
Distortion:
(±3%) less than 1.5%
Light falloff: at f/8
±1 stop from theoretical limit 0.4 stop

Tokina 500mm F/8
PERFORMANCE
Modern Photo's
Standard as Tested
Focal length: ±5%
(475-525mm) 504mm
Aperture: ±5%
(f/7.6-8.4) f/8.3
Distortion:
(±2.5%) 0.9% (pincush.)
Light falloff: at f/8
±1 stop from theoretical limit 0.74 stop
Minolta 500mm F/8
F/no. RESOLUTION
at 1:42 magnification CONTRAST
(30 lines/mm)
Center
Lines/mm Corner
Lines/mm Center % Corner %
8 48 42 44 34

Tokina 500mm F/8
F/no. RESOLUTION
at 1:48 magnification CONTRAST
(30 lines/mm)
Center
Lines/mm Corner
Lines/mm Center % Corner %
8 44 32 34 24

[Raybo]

Good info ivoire!

I have had pretty poor contrast in most of the shots I have taken with it and have been considereing getting a hood for it. Has anyone used a hood with a cheep mirror lens? Does it help much (assuming not shooting into the sun)?

I've made on for my Vivitar.

Here is my brother having a play with the lens with the hood on.



It helps quite a lot.

[MattGunn]

found this on the 55b-55bb:

I'm guessing you got this from Adaptall2.com? There is a lot of usefull info in this site. This is where I found out about the differences between the two versions of the lens. I was interested to see photos from both to see if there are any obvious differences in the IQ as they have not been compared directly on adaptall2.com.

I've made on for my Vivitar.
It helps quite a lot.

Having spent my pocket money on a tamron mirror lens instead of a hood I have been looking for something to that can be easily converted into a hood instead. I have found an empty Golden Syrup tin which appears to be about the right diameter and am in the process of adapting it. What did you use or did you make it from scratch?

[ivoire]

I'm guessing you got this from Adaptall2.com? There is a lot of usefull info in this site. This is where I found out about the differences between the two versions of the lens. I was interested to see photos from both to see if there are any obvious differences in the IQ as they have not been compared directly on adaptall2.com.

I've got some free time over the next 10 days. If anyone with the Tamron 55BB wants to meet to do a test, I'd be up for it. The info I found was on adaptall2.com. They seem to think the 55B is better at distance and the 55BB at macro ("It may turn out that the earlier version is slightly superior for long distance photography while the later version likely is somewhat superior for tele-macro work"). I got the 55B for this reason.

[Marc Sabatella]

When I compare the effective aperture of my Tamron 350/5.6 with any other lens (based on the shutter speed needed to take a normally exposed picture), I find it to be F/8, while my Tamron 500/8 is in fact F/11.

Maybe some brands take into consideration light losses due to the central obstruction and to the less than perfect reflective surfaces of the mirrors but apparently Tamron doesn't. I would be curious to know if you get the same exposure from your Centon than from a refractive lens at the same aperture.

My Vivitar 500/8 definitely comes in at around f/11, too. And from what I've read, that's pretty much universal. f/8 might be the f-stop, more or less, but the "t-stop" is more like f/11 for typical mirror lenses. Meaning that while it might literally be f/8, and DOF might be as expected for f/8, the exposures you get are more consistent with f/11.

Matt's comments on the nature of the reflective coating being the reason for this, and the suggestion that this could potentially be improved, are certainly interesting!