Pentax-K 50mm lens, thread of 49mm or 50mm?

Question

I really want to do some monkey macro photography. I am a student with almost no time and money, and want to take some hobby pictures in my spare time. I have the Canon EOS 450D (Rebel XSi/Kiss X2). My lenses are Canon ZOOM lens EF-S 18-55mm 1:3.5-5.6 IS 58mm and Tamron AF 70-300mm f/4-5.7 Di LD Macro 1:2 Both with Auto and manual focus. Both without manual aperture. I bought an extension tube and 58mm reverse adapter ring for the EF-S 18-55mm canon lens but as you can guess, I can't control the aperture. ( I will try to modify the total €7 tube and ring so it will work ).

Next I have this nice looking SMC Pentax-m 1:2 50mm 5539731 ASAHI OPT. CO. JAPAN I want to use as my reverse macro lens. Inside of my lens cap it says 49mm, on my lens it says 50mm. I just can't find the reverse adapter ring for it. Can someone please help me? (found the reverse adapter. It is 49mm)

Image can be found here of my Pentax lens: image of Pentax lens

After some more searching, I've found a 450D macro reverse ring for 49mm lenses. I will buy that one and see if it works. Do you guys think it will work?

Answer 1

To get a smaller threaded lens to work with a 58mm adaptor just requires the appropriate stepping ring. You can get an entire set of these for not much money, or you can measure the lens to find out exactly which size you need (my money would be on the value stated on the lens itself to be the correct one):

http://www.amazon.co.uk/Fotodiox-Anodized-49-52mm-52-55mm-55-58mm/dp/B001G445Q4

You can also use your 18-55 stopped down if you hold down the depth of field preview button then unmount the lens, this will leave the iris set to whatever value the camera was set to.

Answer 2

I bought an extension tube and 58mm reverse adapter ring for the EF-S 18-55mm canon lens but as you can guess, I can't control the aperture

You can actually control the aperture using the following way:

1. With your lens connected to the camera body, Set your camera to aperture priority and choose an aperture value.
2. After choosing the aperture value, press and hold the DoF preview button found on your camera.
3. While still holding the DoF preview button, remove the lens from your camera.

The lens aperture should be temporarily stuck at that aperture. You can shoot reverse-lens with that now. The aperture opens up again whenever you reconnect the lens to the camera. Don't close the aperture too much, else you'll need lots of light for your pictures.

Disclaimer: Do at your own risk. I've always done it like that though. A photo of a fly I took using this technique.

Answer 3

I just looked at mine to be sure, and Pentax SMC M 50mm f/2 does not have the thread diameter marked on the lens either on the front (as pictured) or on the edge. Many lenses do indicate it usually in combination with a diameter symbol "⌀", but this one doesn't, probably because there's not much room with the writing that's already there. The 50mm is just the focal length. As with most Pentax lenses of this general size and age, the filter diameter is 49mm.

Given that, any 49mm reversing adapter will work.