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 on: March 30, 2016, 04:49:11 PM 
Started by klindup - Last post by Bob Atkins
Mirror lenses can be pretty good - as well as small, light and inexpensive. They can also be not so good.

See my review of the Pro-optic 500/6.3 lens

It's cheap ($160 when tested) and nominally reasonably fast ("f6.3"), but unfortunately it's not all that sharp and the T-stop is closer top f10 than f6.3.

The Tamron 500/8 (discontinued) is much better, plus it's actually smaller and around the same price (if you can find a used one). If you must have a long lens and can live with the limitations of a mirror lens (fixed, slow, aperture plus manual focus and "do-nut" out of focus highlights), it's not a bad choice.

 on: March 29, 2016, 09:20:31 PM 
Started by klindup - Last post by klindup
I take your point about telescopes being so heavy and bulky and that lenses designed to be used on cameras are the best option.  I also have a Televue NP 101 bought for observing but which makes a great 540mm f5.4 lens if the walk from the car is but a few yards.  The weight of the telescope and the Gibralter mount needs two trips to set up and a third to fetch the camera.  However if I can find a suitable site the images I get of birds are great and worth the effort.  But not the first choice when looking for a 500mm lens.

 on: March 29, 2016, 08:56:58 PM 
Started by klindup - Last post by Bob Atkins
To the best of my knowledge, they are both essentially the same thing. They are a negative lens system - which effectively increases the focal length of the primary lens. I'd guess the main difference is that they are designed for different applications. Barlow lenses are typically used visually and don't need a wide field, while TCs are designed for full frame 35mm coverage. There are probably barlow designs that will cover a wide field and there are certainly higher power barlow lenses (up to maybe 4x or 5x), but the principle of the barlow lens is the same as that of a TC.

The answer about SC telescopes vs mirror lenses is again lost in the detailes. They are the same thing optically. Most mirror lenses are SC, though there are, I think, some Rusian Maksutov designs. Lenses will typically have some sort of field flattening optics that some (most) telescopes lack - but you can often buy external field flattening optics for photographic work. For a scope you want maximum visual brightness (minimum number of elements) while for a lens you want maximum field correction over a full 35mm field.

I've used a Teleview 500mm f5 (100mm) refractor as a lens, and it's a great lens. Long, heavy and inconvenient as a lens, but excellent optical quality. I'm sure you could use a Questar for excellent images too, though as you say, it's not exactly a cheap way to go!

So if you have a telescope, it's not a bad idea to try using it as a lens. However if what you want is a lens, then I think you're typically better off buying a lens - and a refractive lens if possible. The only use I see for mirror lenses is if you need something really small and light (e.g. Tamron 500/8 mirror). The only time a mirror telescope might be the instrument of choice is if you want something with a really long focal length. Maybe something like a an f10 8" SC which would give you a 2000mm f10 lens (through I don't know if will cover a full 35mm or APS-C frame).

 on: March 29, 2016, 06:08:35 PM 
Started by klindup - Last post by klindup
Hi Bob
Just read your article on mirror lenses and a couple of questions came to mind.  Instead of using a TC, could you use a Barlow or a Televue Powermate?  Would there be any advantage over a TC?
Second question relates to the lens.  Would you be better using a small SC telescope On an altar mount or even a Maksutov such as a Meade?
I know we are talking unrealistic silly prices in this context but I have seen great results using a Questar. They even used to advertise what you could do using one as a wildlife lens.  Not a realistic option I know on cost basis but it demonstrates what a catadioptric device is capable of.

 on: March 11, 2016, 04:59:44 PM 
Started by claygrazer - Last post by Bob Atkins
As far as I know there is nothing like it anywhere online!

My solution to the issues of upgrading my OS form XP 32-bit to Win7 64-bit was to install the Microsoft Virtual (XP) Machine, which emulates a 32-bit Windows XP system, then run Blurcalc (and a bunch of other older programs that don't work with Win7 64bit) on the Virtual Machine. It works well and allows me to use quite a few older programs that I could never get to work on Win7 64-bit.

You can find the Virtual Machine at

It's pretty unlikely I'll ever get around to updating Blurcalc or Dofcalc to work on Windows 7/8/10 64-bit systems. It might happen, but I wouldn't hold my breath for it.

Blurcalc -
Dofcalc -

 on: March 11, 2016, 04:51:21 PM 
Started by peter45 - Last post by Bob Atkins
My favorite "all around" lens is actually a camera, the SX50, which has an "equivalent" 24-1200mm lens! see

While it clearly can't compete with DSLR image quality (though at ISO 100 it's not that far behind), it's very convenient, very small and light and very flexible. Though the SX50 has gone, the SX60 is similar in concept and function, but with an even large zoom ratio (21-1365) I've shot with one and I liked it. It was more capable and faster than the SX50.

I've also been tempted to take a look at the Canon G3X with a larger 1" sensor and a 24-600/2.8-5.6 lens it seems like it should be a pretty versatile camera. I still haven't had my hands on one though.

I've never boon a huge fan of superzooms for DSLRs. If I had to throw on one lens for general photography it would probably be my EF 24-105/4L IS USM. It's fairly fast, pretty sharp and I can use it with both APSC and Full Frame bodies.

 on: March 11, 2016, 12:21:48 PM 
Started by peter45 - Last post by claygrazer
I really enjoy my EFS 15-85 IS lens.  If I need more reach, I have my 70-200 IS f/4.0 to use.  Both are light enough to carry all day and the resolution on both these lenses is great.  Having one lens with the range you mention tends to compromise the image quality.

 on: March 11, 2016, 12:16:06 PM 
Started by claygrazer - Last post by claygrazer
For us OLD folks that use Bob's old Blurcalc program on XP 32 bit machines, it is a joy.  I've not found anything else like it online.  It would be nice to have a concise source that gives the formulas used in the old Visual Basic Blurcalc program.  If available, they could be put into a newer program for our Bokeh enjoyment.
Glad to be part of the Community!

 on: February 23, 2016, 12:08:10 PM 
Started by klindup - Last post by Bob Atkins
Not in person. I have looked at the specs and compared them with the 70D - - noting the differences and improvements.

Looks to me like the biggest difference for the average shooter in normal operation would be the larger number of AF zones. Personally, 95% of the time I use the center AF zone. I don't think I ever let the camera pick the AF zone. However some people use the outer zones and they can be helpful in tracking a moving subject, so I can see an increased number of zones being useful for them.

The larger number of pixels (24 vs 20MP) really isn't likely to result in a noticable difference. The ISO range is pretty much the same so I don't expect to see much difference in noise and DR. The 80D does add 60fps 1080p video and a headphone jack for monitoring audio, so for videographers those features might be of use. I don't shoot video, so they don't matter to me.

Overall the 80D is a nice incremental upgrade to the 70D, but the difference really isn't enough to make me think about upgrading (and I have a 6D and 7D, so I really don't need yet another DSLR!). If they had given it AF at f8 and changed the sensor design to significantly increase DR and reduce noise and maybe going from 20MP to 30+ MP I'd have considered upgrading. However if that ever happens it will probably take at least 1 or 2 more upgrade cycles to get there.

I'm pretty happy with my 70D and not looking to upgrade. However if I had a Rebel or a 60D or earlier EOS DSLR I'd be looking at the 80D as the best mid-range, mid-priced upgrade. For those looking for a bargain, the 70D price might drop significantly now that it's no longer in production. However right now, the 80D at $1199 ( looks like a better deal than the EOS 70D at $999 (, though you can get an "open box" 70D for $799 (

 on: February 18, 2016, 11:56:38 PM 
Started by klindup - Last post by klindup
Hi Bob
Have you had a chance to look at the new 80D yet?

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