Originally posted by KFBK95 Hi,
Thanks for the info. We want to capture the near-field of a jet (say x/d = 2) and so it is important to reduce the size of the FOV as much as possible.
Carl Zeiss™ Planar T*85 mm f/1.4 is good but the FOV is still a bit large. I also don't think Pentax has 150 mm lenses that are compatible with our camera within the working distance. Do you have any other suggestions?
Tough call.
Longer focal lengths will give you a smaller FOV, but you will loose in terms of depth of field and aperture. Longer lenses are typically f4.8 and above (i.e. smaller aperture and longer depth of field).
The CZ85mm f1.4 (in our case) an optimum compromise between (a) the needs for (a.1) a fast lens (f1.4) for very fast exposure (1/80,000s typically) and (a.3) a shallow depth of field with large aperture (f1.4, giving us typically a depth of field about 3-5 mm) and (b) a small FOV.
While I understand your wish/intent, do not forget that you are likely to find a compromise between a number of different constraints. Thus, do not discard the CZ85mm f1.4 as an option, because 85mm is possibly the longest focal length at f1.4 on the market, irrespective of the brand and manufacturer..
Another direction could be to move your camera (and lens) closer to the jet. I do not know what type of jet and what liquid you use. In some cases, it might not be possible for many reasons. But.... any photographer working with prime lenses know that "your feet are your zoom"
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Hope it may assist. Other might have further good advice.