Thursday, June 11, 2015

Experiments with a different format lens

Note: this is a slight rewrite of the original post to address an 'error' that had crept into my thinking.

All photographers are aware that sensors smaller than a full frame 35mm, come with a ‘crop factor’. For instance, my 50D has a crop of 1.6, meaning that my 500mm full frame focal length lens on my 50D ‘looks’ like an 800mm lens, compared to the 500mm on my full frame 5D3.

For instance, a 645 format (56 x 41.5mm) has 2.6 times the area of full-frame 35mm (36 x 24mm), or a 0.62075 crop factor. In other words, a 100mm lens on 645 sees about the same angle of view as a 60mm lens does on my full frame 5D3. But, see below.

Although there have been adapters for 645 lenses, the latest adapter from FotodioxPro has the advantages of a built in ‘throttle’, ie a 2-10 stop variable ND filter.

So, intrigued with the technology as usual, I found a cheap Mamiya 45mm 645 lens on eBay and purchased the FotodioxPro 645-EOS Throttle.

It all arrived today and first impressions are favourable.

The 45mm Mamiya lens seems 'clean'. The lens is a manual aperture lens, which isn’t an issue as the throttle is passive. Ałso, as I'm not changing sensors, the 45mm Mamiya acts like a 45mm EOS lens on my 5D3.

Using Magic Lantern RAW histogram I was able to set up the desired exposure at, say F/2.8 and an ISO of, say, 400. Then simply ‘correct’ the exposure time for my F/16 image at ISO 100, ie 90s exposure.

Here is a B&W test image, taken in sunshine at 90s.

It’s early days and I’m sure I will find some problems: for example I have a second hand lens that I’m sure has some dust in it, but it seems clean from fungus. I’ll post more feedback in future posts.

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