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Forum: Pentax SLR Lens Discussion 11-07-2017, 03:23 PM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
I hope that's a pun. Aperture... holes?
If so nicely done.
Forum: Pentax SLR Lens Discussion 11-07-2017, 10:00 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
The equivalence of aperture is meant to show that fast lenses on small sensors aren't as fast as they seem when taking ISO limitations of the smaller densely packed sensors into account as well as to show the delta on depth of field. The problem is that sensor density isn't a fixed property nor is ISO performance strictly based on sensor size. For example the K-3 and the K-5 are the same size sensor with very different noise characteristics due to pixel density and technology.

As a heuristic it's a bit of a poor way to compare.

My LX-7 has an f1.4 - 2.3 lens. Technically since I have to shoot at fairly low ISO to get good results this is not as impressive as it sounds - but in practice after trying it against some other cameras it is a bit more practical than the "equivalence" might suggest. On the whole it is better just to understand the ISO performance and DOF of any given platform and take it from there.
Forum: Pentax SLR Lens Discussion 11-06-2017, 07:56 PM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
On MFT there is a bit of truth. The angle of entry to the sensor supposedly made designing wide angle lenses complex due to the close proximity of the rear element to the sensor and the interaction of this with the color filter. This isn't a problem with adapted lenses typically since they are farther from the sensor. But it is true that the digital lenses had to be designed with this angle in mind.
Forum: Pentax SLR Lens Discussion 11-06-2017, 03:14 PM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
Through a quirk of fate I own both the HD DA 70 and the FA 77 Ltd. I find them both appealing. The FA 77 gives a little more subject isolation but it isn't a huge difference. The DA 70 is a bit less prone to CA and purple fringing. They are both great lenses.

Also @monochrome - I think the DA 70 is pretty close to FF performance from what I recall reading. I do not have the K-1 to test on sadly.
Forum: Pentax SLR Lens Discussion 11-06-2017, 08:45 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
Only if you use the K-1 as your source. If you start with the K-3 or K-3ii and you crop the FF to a similar view as what you had in the K-3 and then resize both back to the full uncropped size the K-3 will have more resolution. It is more complicated than x is better than y - but it's also simple - shoot with what you have and enjoy it.
Forum: Pentax SLR Lens Discussion 11-05-2017, 11:01 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
For years when Film was the only medium it was well known that larger format lenses offered lower resolution than smaller format lenses when adapted to smaller size film. People shooting on the Q platform have been talking about this for a while - the native lenses outperform adapted lenses - however when dealing with crop and full frame the difference is so small that I don't think this is typically seen to be a problem.

What I get frustrated by is that he mixes the equivalence and FF vs. crop lenses together. No matter what you think of equivalence - a FF 200 and a crop 200 at the same f/stop give the same image with respect to these factors on the crop body. So the question isn't related to FF vs. Crop LENS - it's related to the format of the sensor and how you frame things.
Forum: Pentax SLR Lens Discussion 11-05-2017, 10:55 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
You lost me - which "tests"? If you mean exposure - then I disagree that's a depth of sorts. Depth is a proxy for volume per unit of area. The instantaneous depth of a moment of light hitting the sensor is essentially the same metric. The depth per moment of time is akin to the number of lux captured. An f/2.8 lens mounted on any sensor is roughly going to make the same basic exposure at the same ISO.
Forum: Pentax SLR Lens Discussion 11-05-2017, 10:35 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
And they both will hold the same DEPTH of water if they started empty - the depth is based on the water falling per square inch.
Forum: Pentax SLR Lens Discussion 11-05-2017, 09:14 AM  
Full-frame lens on a crop body not advisable?
Posted By UncleVanya
Replies: 168
Views: 13,005
The truth is this. If you use a Full Frame lens on your crop body you are using a lens that has been designed for a larger image circle than you have a need for. This means the lens is larger than needed. It is also true the resolution of some older full frame lenses may not be as high as that of more modern crop (and full frame) lenses but that is of very little impact.

As for light gathering - as someone commented f/x.y is the same no matter what the lens image circle is. It is true you are "throwing away light" the light outside the sensor - but that has no impact on your exposure. The light per square mm is the same either way. The consequence of the light being thrown away is that the lens is heavier and larger than needed but that's all.

This same set of criteria is also in play when mounting lenses from full frame or crop on Micro 4/3 or from 645 to 35mm etc. The larger image circle lenses are just heavy and bulkier than the lenses made for the exact image circle needed.

The main benefits to using a larger image circle lens is sharing the lens over multiple systems or that corner performance and vignetting will be improved (typically) when using a lens designed for a larger image circle.
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