Shooting with a CAT

[jemann]

Hiya folks.

I recently borrowed a Sigma 600mm f/8 catadioptric lens and have been trying to take some moon shots with it. I've not overly impressed with the results, the best of which is here [flickr]. The 1024x1024 image is a 1:1 crop of the original.

I know cat lenses aren't known for being particularly sharp, but I've seen much better results on various websites, although these tend to be daylight shots of things like birds of prey.

I was wondering if anyone could give any tips on how I might improve the sharpness of my shots with this lens. I was shooting at ISO 100 with various shutter speeds between 1s and 1/100s. The camera was tripod mounted and I used mirror lockup and remote release.

Am I expecting too much from this lens? Or do I just need to change the way I shoot?

Thanks for any help.

Jason

[Marnault]

Looking at the image of the moon I would say it was a bit out of focus the details should be a fair bit sharper than that at 600mm. The reviews I have seen for that lens are pretty good considering its a mirror lens. It could be out of collimation if you cant get a better focus than that, I'm not sure if you are able to correct this your self or need to send it in, I know on Catadioptric telescopes there are usually collimation screws which you can adjust.

As for using a mirror lens, the biggest thing you need to pay attention to is your backgrounds, mirror lens's have a unpleasing bokeh which looks like a donut, so you generally want the background to be fairly uniform and no bright spots.

Another thing to try is to increase the shutter speed, if your tripod isn't all that stable even the smallest vibration can cause some blur at 600mm, you said you used up to 1/100, this should be pretty good, but just in case try bumping up the iso to 400 or even 800 and increase the shutter speed.

Another disadvantage I have noticed with a mirror lens is that the images usually lack contrast when compared to the same image with a normal lens so some extra post processing or idealy shooting two images with slightly differant exposure settings (one for highlights, one for shadows) and combine them in photoshop.

I have a 1600mm F12.1 manual focus Catadioptric lens (well its actualy a telescope, only real differance), a fair bit longer than yours but its still a mirror lens. I have had some success with this lens, biggest problem is the weight, it takes a pretty good mount to keep it steady, and for astrophotography you need it to extremely accurately compensate for the rotation of the earth which can be quite tricky. I have only used it a couple times for normal imaging, just to shoot commercial jets from long range really which it has worked fairly well for.

Here is a image of the moon I shot a while back with it (its actually a composition of two images because the whole moon didn't fit in the limited field of view)

[KirkHMB]

I'll agree with focus. I have an old FD 500 f8, and bought a focus magnifier to help me really see the screen well. Really helped with the OOF problems.

[jemann]

Thanks for your comments.

I must admit, it was pretty difficult to manually focus that lens. I got it as sharp as I could through the viewfinder, but it's pretty small on the 400D.

I wonder, does the distance between one's eye and the viewfinder affect how focussed something appears to be? Usually I wear my contacts when shooting, but on this occasion I was wearing glasses and so my eye was about 1cm further away from the viewfinder.

[PhotosGuy]

1s and 1/100s.

ISO 100 at f/8 would give you an exposure of about 1/250 sec. I'd have tried ISO 400 @ 1/1000 & the pic probably would have been sharper.

Manila Moon at 1120 mm (large pic)

[KirkHMB]

Originally Posted by jemann

T
I wonder, does the distance between one's eye and the viewfinder affect how focussed something appears to be? Usually I wear my contacts when shooting, but on this occasion I was wearing glasses and so my eye was about 1cm further away from the viewfinder.

I don't believe distance from the finder makes a difference, but are your contacts and glasses the same prescription? (Mine aren't, I can't see up close with my contacts, wear bifocal glasses). I can't manually focus with my contacts, I can barely read the LCD at times. Does you camera have diopter adjustment on the viewfinder, if so, adjust it for best.

Take your time, take many shots with micro adjustments to the focus, review on the LCD with magnification till you see one you like. Not rocket science, but the film is cheap.

[PhotosGuy]

Does you camera have diopter adjustment on the viewfinder, if so, adjust it for best.

It should have one. And mark it so you can tell if it's been moved at a later time.