DirtFarmer Loc: Escaped from the NYC area, back to MA

One right answer above.
The pixel count is irrelevant.
The "crop factor" is determined by the physical sensor size.
If the crop factor is 1.23 then the focal length required is 246. But crop factors vary widely.

Why don't you just use a zoom lens?

blackest Loc: Ireland

Ok so we have established that a 250mm focal length on the full frame can crop out an image 6000 by 4000 px with the same image as a d7200 with a 200mm lens.

The images are the same as they can be the detail is largely a match but there is a difference the 6000 by 4000 image in the d7200 is physically 24mm by 16mm and the K1 sensor is 36 by 24 (7360 x 4912) however we have applied a crop (6000/ 7360 x 36mm) 29.3mm by (4000/4912 x 24mm) 19.5mm. Clearly 29.3 x 19.5 (571 square mm) is bigger than 24x 16 (384 square mm) The sensor area used to get the same resolution on the crop is around 50% larger.

So with the full frame and a 250mm lens v a aps-c with a 200mm lens.
you get flexibility in cropping and need to enlarge less for any given print size
There will be less noise in the full frame sensors pixels
At reasonable print sizes the results will be near identical and when they are large enough for pixel sizes to make a difference the crop sensor will break down first.

BebuLamar

Not any full frame, it's specific to your K1. If you use the Nikon D850, Canon 5DSR it will be different.

Longshadow Loc: Audubon, PA, United States

That's why I don't worry about pixels, just the way the image is composed (and exposed, but that's another subject).

BebuLamar

OP said "Mathematical Challenge" so I am only interested in the math.

User ID

`

The 1.23X crop and the 245mm is quite correct,
and many respondents understand this. Those
respondents who figure otherwise need to more
carefully READ THE OP ! You will find this is not
the usual, way-too-often-asked, basic "FF:APSC
FL equivalency" question.

.

blackest Loc: Ireland

True D850 is 45Mpix with around 20Mpix crop.
Without doing the math less than 250mm lens to match a 200mm image on a d7200, again that image will be bigger than aps-c sized and will need less enlargement for any given print size.

Thats if you care about having less pixels in a cameras crop mode, personally i think it should be fine i have no issues with 16 mpix crop or even my 10.2 Mpix k200D. my old 1D mark II is not too bad either.

ggab Loc: ?

"The crop factor is not 1.23 it is 1.5 just like any full frame vs Nikon dx."

ggab Loc: ?

"a k1 is 36Mpix full frame and a d7200 is 24Mpix aps-c.
If I put the K1 in Aps-c mode then it will have the same angle of view as the D7200 with a 200mm lens but it will be a little less than 16 Mpix in size. So the fella with the D7200 says he has more megapixels in his image since he has 24 Mpix.

Ok so now I crop my photo to 6000x4000 same as the D7200 but the field of view is wider. So I need to pick another focal length that gives me the same field of view as the d7200 when I crop to 6000x4000".

I believe you are forgetting to take into account the pixel size of the sensor in each camera. While you can put the K1 in APS-C mode, presenting a field of view comparable to the D7200, the pixel size of the sensor of the D7200 is much smaller. This provides greater pixel count for the same field of view.

"Pixel pitch is a measure of the distance between pixels".
"D7200 Approximate Pixel Pitch: 3.92 microns" "Pentax K1 Pixel pitch is 4.86 microns". If we assume the same sensor area, the D7200 will have more pixels.

Because of this, I believe the crop factor is 1.5 and not 1.23.

Disclaimer: I of course could have all of this wrong in which case I apologize.

digit-up Loc: Flushing, Michigan

Is that the attitude you want us to know you by? Unfriendly!! RJM

Rich1939 Loc: Pike County Penna.

Whatever FF lens you put on your K1 it will project an image that covers the entire frame. When it does that your saved image will be 7320 pixels wide. If you switch to DX mode on the K1 the saved image will be considerably less than 6000 pixels wide. (I couldn't find the numbers in the spec sheets.) In other words you cannot crop in camera to a 6000 wide image. You will have to determine where those borders are in your view finder (or live view)and using a zoom lens,compose the image you want so it falls within that and do a final crop in post.

Longshadow Loc: Audubon, PA, United States

gvarner Loc: Central Oregon Coast

Maybe start by setting your D7200 crop factor to 1.3 to see the affect?

Mercer Loc: Houston, TX, USA

Not particularly, but whatever rings your chime is clearly the right thing for you. Personally, I prefer the tried and proven WeeGee method as a basis: "F11 and Shoot."

lamiaceae Loc: San Luis Obispo County, CA

The pixels of the two cameras are not the same size. So one probably should be comparing the physical sizes of the two cameras formats. The Nikon FF or FX crop factor (none really) is 1x. The Nikon & Pentax APS-C (DX or Crop Factor) crop factor is 1.54x.

So I'd guess, 308mm. I'm not wasting any more time on this. I know what each of my lenses do on each of my cameras, so I don't endlessly convert numbers. This includes my 90mm, 120mm, 150mm, 210mm View Camera (Large Format) Lenses.