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09-01-2010, 07:46 PM   #1
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Full frame/APS-C conversion chart for dummies

I made up this chart for myself so that I wouldn't have to keep doing the math when choosing lenses for my k10. It is based on the 1.5 crop factor/focal length multiplier. I only recently made the jump from film to digital and have notes stuck all over the place to remind me of this, that and the other thing as I try to learn everything at once. I started out with a k1000 back in the 70's but shooting digital requires me to "go back to school". Because of my age, I don't think it matters if I use this "cheat-sheet"!
Enjoy and please let me know if any of you have suggestions that might make this list more comprehensible.

PENTAX LENS FOCAL LENGTH CONVERSION CHART
Full-frame >APS-C size. Using 1.5 crop factor.

6mm > 9mm / 38mm > 57mm / 70mm > 105mm
8mm > 12mm / 40mm > 60mm / 72mm > 108mm
10mm > 15mm / 42mm > 63mm / 74mm > 111mm
12mm > 18mm / 44mm > 66mm / 75mm > 112.5mm
14mm > 21mm / 46mm > 69mm / 76mm > 114mm
15mm > 22.5mm / 48mm > 72mm / 78mm > 117mm
16mm > 24mm / 50mm > 75mm / 80mm > 120mm
18mm > 27mm / 52mm > 78mm / 82mm > 123mm
20mm > 30mm / 53mm > 79.5mm / 84mm > 126mm
22mm > 33mm / 54mm > 81mm / 86mm > 129mm
24mm > 36mm / 55mm > 82.5mm / 88mm > 132mm
26mm > 39mm / 56mm > 84mm / 90mm > 135mm
28mm > 42mm / 58mm > 87mm /
30mm > 45mm / 60mm > 90mm /
32mm > 48mm / 62mm > 93mm /
35mm > 52.5mm / 64mm > 96mm /
36mm > 54mm / 66mm > 99mm /
68mm > 102mm /

100mm > 150mm
105mm > 157.5mm
135mm > 202.5mm
200mm > 300mm
210mm > 315mm
250mm > 375mm
300mm > 450mm
320mm > 480mm


Last edited by jonhock; 09-02-2010 at 09:59 PM. Reason: info didn't make it
09-01-2010, 07:54 PM   #2
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What chart? Can't you just x1.5?
09-01-2010, 08:17 PM   #3
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Sure you can! I just like to have the info all in front of me when I'm setting up a shot. This helps me to reach for the right lens the first time without having to think; ok, if this times that is this, then that would be almost 80 but if I use this, wait, i'm calculating here.......

I know that my 18mm to 55mm acts like a 27-82.5mm but if I forget then I have to do the math quick before that bird flies away and I realize I needed my 28(42)mm to 80(120)mm. With the chart pinned to my camera bag, sooner or later I'll have all the calculated focal lengths of my lenses memorized. And then/also I can use the chart while shopping and with my LBA I need to know NOW!
09-01-2010, 11:51 PM   #4
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Seems like *much* more work to have to look something up on a chart than to multiply or divide. And in any case, soon enough you won't need a chart, as you'll simply think in APS-C terms. But in any case, sure, it looks like you've correctly multiplied by 1.5 a few dozen times there. Do you actually own own lenses in all those focal lengths? Wouldn't it be far simpler to just have conversions for the lenses you own? Who cares what that 35mm equivalent of a 56mm lens is on APS-C if you don't own a 56mm lens? I think you're making this far more complicated than it need be.

09-02-2010, 12:29 AM   #5
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It's actually a division factor of EXACTLY 0.65 on Pentax's sensor, not a multiplication factor of 1.5. If you want to figure out DOF and FOV, you'll be off by 4% with 1.5. For instance: 50mm is 77mm not 75mm.
09-02-2010, 05:46 AM   #6
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Do ya think I'm over-thinkin what I'm thinkin?

This is what the card looks like-see attachment.
As I said, I made this for myself, just thought I'd share it.
On the card the "normal" focal lengths are in bold print.
No, Mark, I don't have a 56, but I do have a 58mm HELIOS-44M-4. If you look closely you'll see that I didn't do that much math, just add 3 to each 1.5 length. My lenses are-18,28,35,50,58,70,75,80,200, and 300mm (including zooms) and as I said, sooner or later I'll just "know" without having to do math in my head OR look at the "chart".

In my quest for knowledge I always remember one thing "Even a blind squirrel finds a nut once in a while".
Attached Images
File Type: pdf PENTAX LENS FOCAL LENGTH CONVERSION CHART.pdf (33.4 KB, 1275 views)
09-02-2010, 01:23 PM   #7
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QuoteOriginally posted by troglodyte Quote
It's actually a division factor of EXACTLY 0.65 on Pentax's sensor, not a multiplication factor of 1.5. If you want to figure out DOF and FOV, you'll be off by 4% with 1.5. For instance: 50mm is 77mm not 75mm.
Hey there, Trog.
Where did you get those figures at? Maybe I can make another chart..........
09-02-2010, 02:12 PM - 1 Like   #8
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0.65 * 36mm = 23.4mm. 0.65 * 24mm = 15.6mm. The Pentax APS-C sensor is 23.4mm x 15.6mm.

09-02-2010, 04:06 PM   #9
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It takes that slow for you to divide/multiply by 1.5?

QuoteOriginally posted by jonhock Quote
Sure you can! I just like to have the info all in front of me when I'm setting up a shot. This helps me to reach for the right lens the first time without having to think; ok, if this times that is this, then that would be almost 80 but if I use this, wait, i'm calculating here.......

I know that my 18mm to 55mm acts like a 27-82.5mm but if I forget then I have to do the math quick before that bird flies away and I realize I needed my 28(42)mm to 80(120)mm. With the chart pinned to my camera bag, sooner or later I'll have all the calculated focal lengths of my lenses memorized. And then/also I can use the chart while shopping and with my LBA I need to know NOW!
09-02-2010, 04:33 PM   #10
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QuoteOriginally posted by troglodyte Quote
0.65 * 36mm = 23.4mm. 0.65 * 24mm = 15.6mm. The Pentax APS-C sensor is 23.4mm x 15.6mm.
It certainly is. That makes the Pentax formatfaktor 1.54, not the 1.44 of a 16.7mm x 25.1mm APS-C sensor. On one hand, using a formatfaktor of 1.5 for *ANY* APS-C camera is wrong. On the other hand, 1.54 (Pentax) is closer than 1.44 (APS-C) to the common 1.5 formatfaktor. But on the gripping hand, does the human eye distinguish a 5% variance in image size, without pixel-peeping closeness? In other words, does it matter in real-world application?

QuoteOriginally posted by vtqanh Quote
It takes that slow for you to divide/multiply by 1.5?
Ah, but that's not close enough. Try mentally multiplying/dividing by 1.54, eh?
09-02-2010, 05:18 PM   #11
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There is no 21, 31, 43 or 77 on the chart, so we can't use the Three Amigos and his brother :-(
09-02-2010, 09:56 PM   #12
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they're hiding

QuoteOriginally posted by SpecialK Quote
There is no 21, 31, 43 or 77 on the chart, so we can't use the Three Amigos and his brother :-(
Since the even numbers (22,24-30,32 etc.) jump 3mm, then an odd number lens like these four "limiteds" would only jump 1.5mm (half of 3) from the next lowest even number. Unless you're shooting on the north pole, then it's a whole new formula.
Oh, and vtqanh, it doesn't take me long to figure one focal lenth but if it's a zoom, I forget the first mm by the time I figure up the second one.
Out of all the numbers I've seen online and in the forum, the "1.5" seems to be the most often mentioned so I'll stick to that. But, I will keep in mind (and write it on my paper) that it's not as precise as Trog's info which is pure common sense. As far as DOF & FOV, Ole has those points covered pretty well in his threads; "Crop Factor, Focal Length and Field of View"
"Field of View Tables, APS-C, 24x36, 645D, 645, 6x7" and "Field of View, Full Frame and APS-C compared"


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09-03-2010, 01:36 PM   #13
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Also, for that matter, the stated focal lengths on lenses are seldom all that accurate. And it's not like anyone really has a clear notion in their head of what a 70 versus 72 looks like - even if they owned lenses in both focal lengths (which no one probably does). To me, that's just wasted clutter in the chart - you're making it take longer than it has any right to just to *find* your favorite lens in the chart. It would take all of about 15 minutes to just memorize the equivalents for the lenses you have.

Last edited by Marc Sabatella; 09-03-2010 at 07:34 PM.
09-03-2010, 05:40 PM   #14
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Perhaps my joke is a bit too subtle.
09-03-2010, 05:57 PM   #15
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jonhock, although I personally wouldn't choose to use a chart or table, I just spent the couple minutes I had while my 4 year-old was brushing his teeth to make this quick chart for you. I thought it might be more useful than a large table of numbers.

EDIT: added a version with red lines for the Three Amigos (SpecialK, I wasn't sure who the "brother" was)


Last edited by dgaies; 09-03-2010 at 09:08 PM. Reason: added version for the Three Amigos
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