[osv]

The short and sweet version: focal length is a physical property and remains the same, which means that DOF stays the same. FOV does change when using a lens designed for a 35mm image circle on a crop-factor camera.

"As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors."
Digital Camera Sensor Sizes: How it Influences Your Photography

[clackers]

I'm interested in this assertion.
Where did you read about it or what led you to believe this?

Several times in this very forum, LensBeginner! I may be repeating an urban myth - that the DA50 is a reworking of the FA50, and the DA35 of the FA35. Someone even speculated that the slightly different aperture is due to some of the glass area needing to make way for thicker plastic mounting, but that seemed dubious to me.


The DA50 has the same six elements in five groups, but seven rounded blades for nicer bokeh.

I've read on an "unspeakable" website that the f/1.7 is "almost as if this was optimized as an f/2 lens" and was oversized to f/1.7 only for marketing purposes, so it all looks plausible...
I can confirm this having tested myself, the horror of the corners at f/1.7, on APS-C... I just wonder what the corners must look like on film or FF... brrr...

But if you look at MTF charts, nearly all those fifties from all brands have similar curves ... they're at their sharpest at f5.6 or f8. And this suited the intended purpose fine ... on film, they were the photojournalism lens of choice, with the depth of field requirements to go with it.


Wide open, only the centre is sharp, and as you stop down, everything gets sharper, particularly the edges.


I have an M 50mm f1.4, and can't use it wide open except to get an artsy isolation of part of a subject. Stopped down the sharpness is like my DA50, even if the rendering's different.

[Jannis]

Thank for your replyies. So, it seems to be good glass, that DA50/1.8.

If you dig around on eBay and you're patient you can nab a mint M 1.7 for much less than $50. I got mine like new with both caps and a small case for $30, it's just a matter of waiting for the right deal!

Yes, I know that on ebay and here on marketplace are good deals, but when I'm from EU I need to pay tax and duty (and shipping) for anything above about $25... So if it is $40 and shipping is minnimally $18 I will pay $70

my recommendation would be to get a Pentax F(a) 50mm macro if your budget allows it.

Thanks for recommendation, but I'm thinking about new lens becouse lack of fast glass. I have f2.8. And I have macro lens allready...

[LensBeginner]

Several times in this very forum, LensBeginner! I may be repeating an urban myth - that the DA50 is a reworking of the FA50, and the DA35 of the FA35. Someone even speculated that the slightly different aperture is due to some of the glass area needing to make way for thicker plastic mounting, but that seemed dubious to me.


The DA50 has the same six elements in five groups, but seven rounded blades for nicer bokeh.


But if you look at MTF charts, nearly all those fifties from all brands have similar curves ... they're at their sharpest at f5.6 or f8. And this suited the intended purpose fine ... on film, they were the photojournalism lens of choice, with the depth of field requirements to go with it.


Wide open, only the centre is sharp, and as you stop down, everything gets sharper, particularly the edges.


I have an M 50mm f1.4, and can't use it wide open except to get an artsy isolation of part of a subject. Stopped down the sharpness is like my DA50, even if the rendering's different.

Thanks for the info.
Yeah, pretty much my conclusions as well...

[ogl]

It's great lens. Really. I've bought it this August and I was impressed. AF is good. Bokeh and colours and resolution are very good.

[stevebrot]

I own:

• DA 50/1.8
• A 50/1.7
• M 50/1.7
• K 55/1.8
• ST 55/1.8
• About a dozen other fast 50s

My impression of the DA 50/1.8 is that it is quite possibly the most fun of any lens I own on the K-3 and even more fun on my friends K-50. It is a capable optic in almost every way and is my first choice when fielding inquiries from noobs about buying a prime lens.

'nuff said

Beyond that, in terms of absolute performance, I would put it a notch behind the M 50/1.7 and K 55/1.8. The reason is hard to fully describe and I have no quantitative data*, but my impression is that DA 50/1.8 lags a little in sharpness and/or contrast. It also tends to glow a little more than I like in bright light and wider apertures. Now that I have that out, here are a few bullet points:

• Works great on the K-3
• Obvious choice as a learning tool for portrait work
• Usable on 35mm FF, though with significantly more vignette at wider apertures than the A 50/1.7**
• The lack of a distance scale is bothersome when attempting focus pull for video. Tape markers might be a good option there.
• I love the smooth manual focus
• Admirable resistance to purple fringing***

I would say it stands up very well in the company of my other 50s.


Steve

* At some point I am going to do a shoot-out comparison with actual resolution numbers for the DA vs. M vs. K

** I tested the two using 35mm film. The vignette is visible on the negative. There is much speculation and several claims regarding the DA 50 and its relationship to other Pentax lenses. This much is obvious...It shares the same design heritage as other moderately fast Pentax 50s. I don't believe it is an AF redux of the K-55/1.8, nor is it a FA 50/1.7 in cheap DA clothing. It is betrayed by its physical dimensions and vignette wide open.

*** This I did test. Try as I might, I could not get the DA 50/1.8 to fringe, even with +2 stops flash exposure of crumpled aluminum foil wide open.

---------- Post added 11-13-14 at 08:39 PM ----------

But if you look at MTF charts, nearly all those fifties from all brands have similar curves ...

That is because almost all Japanese fast 50s for SLRs trace their design lineage to the Zeiss Planar.


Steve

[axelm7]

"As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors."
Digital Camera Sensor Sizes: How it Influences Your Photography

I should have clarified this in my post. A FF lens creates thinner depths of field at equal FOVs because the focal length increases.

[GateCityRadio]

I should have clarified this in my post. A FF lens creates thinner depths of field at equal FOVs because the focal length increases.

A 50mm FF lens and a 50mm APS-C lens won't render any differently on a APS-C body. They are both 75mm equiv lenses on a APS-C body and will have the same DOF on a APS-C body (at the same f-stop). A 50mm lens is a 50mm lens, no matter what size sensor it is made for.

[osv]

since the focal length doesn't change, you don't have more "reach" with aps-c, either... it's just a change in the fov, period.

where people get confused is when the pixel density of the sensors is different... they call more pixels in the same fov more "reach", which it isn't, it's just higher resolution.

i'm not seeing how higher resolution is going to affect dof, either.

[LensBeginner]

since the focal length doesn't change, you don't have more "reach" with aps-c, either... it's just a change in the fov, period.

where people get confused is when the pixel density of the sensors is different... they call more pixels in the same fov more "reach", which it isn't, it's just higher resolution.

i'm not seeing how higher resolution is going to affect dof, either.

I usually write it in inverted commas when I do, and the add "so to speak" or a parenthetical statement to that effect.
If I shoot birds, then more resolution in the same FoV equals more "reach", so to speak, because I can crop more in post (or take advantage of the crop of the sensor).
I know it's different than having a longer focal lenght, because in that case your position with respect to the subject would change, and thus the relation between the various planes ("compressed" depth etc.)
DoF will be affected only if you blow up different sensor sizes to the same print size, as circle of confusions will be magnified differently.

I'm sure you already knew & considered all of this, but I just repeated the obvious because it still makes sense to use certain shorthands while writing, even though we all know what they entail.

[filoxophy]

A 50mm FF lens and a 50mm APS-C lens won't render any differently on a APS-C body. They are both 75mm equiv lenses on a APS-C body and will have the same DOF on a APS-C body (at the same f-stop). A 50mm lens is a 50mm lens, no matter what size sensor it is made for.

At first I couldn't tell if our friend axelm7 was just using the same terminology in a different way. But up in post #17 he agreed that focal length is an inherent physical property of a lens, while then telling me that two different 55mm lenses would not look the same when mounted on an APS-C camera. Oh well, all it would take is for him to perform his own experiment and mount the DA 50mm and a K/M/A/F/FA/DFA/645/6x7-or-any-other-format 50mm and compare the view. It's not a big deal, so long as the OP is not convinced that they should start comparing the DA 50 with the FA 77 because they'll have the same FOV on a Pentax APS-C camera, or something like that.

[GateCityRadio]

I had both the DA50 and M50/1.7....and IMO the DA50 was better. It rendereds better bokeh and the "glow" and "softness" at wider apertures is great for portraiture, which is what this lens shines at on APS-C.







I didn't really notice terrible vignette on my attempts with film on the DA50, a few examples on Tmax 100:

[filoxophy]

Nice shots, Gate.

[kh1234567890]

DoF will be affected only if you blow up different sensor sizes to the same print size, as circle of confusions will be magnified differently

Even that is debatable in practice - view this shot from 12" distance and then from 18". Is there any change in DoF ?

[stevebrot]

It rendereds better bokeh and the "glow" and "softness" at wider apertures is great for portraiture, which is what this lens shines at on APS-C.

Beautiful shots! That being said, "glow" at wider apertures is generally considered to be a fault, not an attribute, and is one of my minor complaints about the DA 50/1.8.* Fortunately it is a problem mostly in strong light and is less a problem otherwise. As for the bokeh, I have not been particularly wow'ed except that that iris outline is a bit nicer at wider apertures due to the curved blades.

BTW...your shots above don't show the glow I am concerned about.


Steve

* There are lenses that are designed for that effect and are usually labeled as such (e.g. "Soft"). I also have a couple of lenses that glow quite readily, but I always qualify that property before recommending them for general use.

---------- Post added 11-16-14 at 03:42 PM ----------

as circle of confusions will be magnified differently

"Circle of confusion" is not something you see per se. In practice, it is a semi-arbitrary value applied as part of the DOF calculation. It represents the diameter of acceptable blur for a point source at given distance from true focus. At various times different lens makers (most notably Zeiss and Leica) have used different values for calculating the DOF scales on their lenses even though the format might be the same.

Magnification is part of the DOF picture and in fact magnification of the final viewed image is one of the three factors that do not cancel out. The other two are absolute aperture and viewing distance. Focal length and capture format are important, but ultimately incidental.


Steve

---------- Post added 11-16-14 at 03:58 PM ----------

Even that is debatable in practice - view this shot from 12" distance and then from 18". Is there any change in DoF ?

I love it when people other than myself point this out.

Here is another test:

Compare this image to my avatar. Which has greater DOF?



Yes, both are derived from the same original file.


Steve