Pentax 645 Lenses on a GFX body.
Fujifilm GFX 100s with Pentax-A 645 75mm f2.8
Fujifilm GFX 100s with Fujinon GF 45mm-100mm @76.5mm
I don’t like forums. It seems a vast majority of the information on them leaves me more confused than when I arrived. I put this down to men (and it is always men) on these forums delighting at being deliberately obtuse.
Since I purchased my first Fujifilm GFX I have been adapting the lenses to the body, I have an adaptor for my 25 year old Mamiya RZ lenses which is great for close up work but due to the size is only really useful for studio work (see the image below), an adaptor for Nikon lenses which is pretty pointless and i only use it to attach my GFX to our telescope.
Fujifilm GFX with Mamiya RZ 180mm + No 1 extension tube and Fotodiox RX to GFX adaptor
I also have an adaptor for Pentax 645 lenses to the GFX and it is this that has been causing me consternation with forums. I bought my first lens for close up work in the field; the Pentax-A 120mm f4 Macro with a Kipon helicoid adaptor and was so impressed with it I bought a Pentax-A 645 75mm f2.8 and Fotodiox Tilt shift adaptor for architecture and then a PENTAX-A 645 35mm f/3.5 also for architecture all from Japan. A while later I saw a SMC Pentax-FA 645 400mm F5.6 ED IF lens for about £300.
The 120mm and 75mm are both excellent and are very useful. The 35mm is a handy small lens to carry if I do not want or need to carry the Fujifilm GF30mm f5.6 TS. The Fujifilm lens is superior to the Pentax mostly due to the Fotodiox adaptor not allowing independent tilt shift and the wider field of view. The 400mm is big shakey and slow to use. With all the lenses particularly the 400mm it’s essential to set up the in body stabilisation, doing this was the first time I started wondering about the focal length… sorry field of view of the lenses… or is it focal length?
I am really going round the houses to get to my point. I wanted to know exactly what the equivilent Fujifilm lens would be to the lenses I had. I thought like many people on the internet the question was “what is the equivilent focal length of the Pentax 645 xmm for a Fujifilm GFX?” I was confronted by similar questions asked on forums with obtuse responses and further responses talking about Crop factor and Field of view with formulas to calculate diagonals and factors. Some of the responses were trying to be helpful and from a place of helpfulness but so many first responses felt like they were stamped out on keyboards by men who in my head sounded like Alan Partridge and had been waiting for the question so they could repeat, “Focal length is focul length is focul length…”
I know this answer is literally correct but when people started talking about crop factors I decided to just shoot a test with my 45-100 and try and work out what the field of view, crop etc my 75mm Pentax lens showed/reproduced?!
As you can see the images above are almost the same and the confusing thing is I thought I had worked it out I thought the GFX would be cropped as its sensor was smaller than the image area of the Pentax. It was cropped in but no matter what calculation I tried it bore absolutely no similarity ot the result I expected. The Meta data shows the Image shot on the GFX had a focul length of 76.5mm? This has left me totally confused by the whole affair. Likewise the Pentax lens 120mm gave me the same image as the Fujinon GF 100-200mm in the 120mm zoom position.
So I think I’ve worked it out and my main misunderstanding was brought about by not having a Pentax 645 to gauge the focal length and actually never using a 645 camera. There are a number of calculations when getting my head around the image I would photograph from different cameras and lenses on different sensor/film sizes.
So the focal length is the same 400, 120, 75, 35 but the 120 on a Pentax 35mm equivalent is 75mm where as the 120 on the GFX 35mm equivalent is 95mm,
This site below, as mentioned by people on the forums is the most helpful.
From Digital Transitions Photo
If you Set the top left purple button to GFX 100 and the below Green button to 645 film and “auto match” to focal length then you can quickly see what I think most people are looking for… I think but I might still be wrong!
If you want to know which focus length in the Pentax 645 lens range to look for, click on “Diagonal” and set the green 645 button to the focal length you want it will show you in the GFX button what Pentax focal length would give the same “view”.
It is all very simple once worked out but my head was all over the place getting there. This is why for me practical experience is the only way to get to the bottom of these things it is also why it takes me 5 times as long to work things out.
It may be I am the only person who didn’t understand the answers on the forums but if not then this is for you, and to all the obtuse guys on the forums, try and read between the lines it will make photography more fun for everyone. x