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06-05-2012, 03:46 PM   #1
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Another aperture/bokeh lens question

I am developing a collection of lenses.

I am also developing an addiction to buying lenses.

Its weird. I think I am becoming obsessive about the aperture capabilities of each lens...

I have some of the telephoto lenses such as the current auto focus 55-300mm as seen on pentax website...

I don't know why--- but if you are at the 55mm zoom (not extended) the aperture will be allowed to go down to 4.0...but if you zoom out to 300mm it will not allow you to go down to 4.0. It will automatically shift to 5.8. Maybe its some auto setting? Is there some way to over ride this auto function so that I can get 4.0 at a 300mm zoom? I have tried a bunch of different settings and still...zoom to 300mm and the aperture is 5.8.

I do not know what the nexus is between focal length and aperture...that would be something to look at.

What I am thinking I want to do is collect some more lenses. I know, I know... but I have a purpose!!!

I am becoming a bokeh addict. I don't mind the lenses being manual. In ways that's a good thing because it forces me to take time to manually compose what I am shooting...and it gives me control... (now that I have some gear I need to learn fundamentals of composing photos and having creative effect and so on and so forth)

I am beginning to think manual lenses allow MORE creativity than the newer ones...(that is unless I can over ride the auto functions)...

Now I have a ricoh f1.4 50mm... I like that one.

Also I have 2 seperate 50mm f1.7 lenses

I have an 85mm f1.4 (rokinon) coming...

I am thinking that if I get an older manual zoom lens that has bigger aperture settings I can have control over things more...

Say for example if I can find a 105 mm lens with a f2.0 or better....

Or a 135mm lens with a f2.0 or better (or the best I can get)...

I am thinking that if I go the longer lens route with a broader range of f stops I can then have a 135mm lens with a 2.0 or 2.5 (or the best I can get) and the camera has to live with what I set. Understand? Get the drift?

If the lenses were fixed focal length that's even better but zooms will work too at least in my theoretical concept.

Maybe I am incorrect but I am directly correlating bokeh capability to the f.stop numbers...But I would specifically want those lenses so that I have the broadest range of apertures on my lenses all at different mm focal lengths. And if they are manual lenses I turn that aperture ring to 2.8 @ 100mm (as an example) then I can adjust the other stuff as needed...

I am a rank amateur so I might be completely wrong here in how I am conceptualizing things.

I have a 28mm f 2.8 but I find that lens only creates marginal bokeh.

If I could go the other direction too... say a 28mm lens with a 1.4 or 1.8 or whatever the best I can get in manual lenses...

I have even heard of people using macro lenses to take regular pictures with...for the exact reason I am talking about...

I have never laid hands on a macro lens so I don't know what's different about those...

Here is a link to a thread that is along the theme I am talking about...

https://www.pentaxforums.com/forums/post-your-photos/137392-misc-100mm-f-2-8-...keh-bokeh.html

Honestly I wish they just made lenses that had the broadest range of apertures...

I don't understand WHY at 300mm my lens has to be at f5.8....

I know I am jumping around here but I am really just learning photography. I can only imagine back in the old days people had to manually do everything then keep notes and then go home and develop film and so forth...something about mixing that with modern technology is appealing. It will make me slow down and figure out what I am doing but when I snap the shutter I get instant feedback.

All that auto focusing and auto this and auto that is great and is very useful but sometimes it seems like it is getting in the way of MY intent which is to learn fundamentals, have control, slow down and be creative...

Right now at times with these auto lenses I feel like I have an over glorified point and shoot. Its not exactly like that...but with some things yes...

I am not getting everything in my head out there, but I am hoping someone understands what I am trying to say and will help start a discussion on it....

For example if you look at Pentax's website they have a 300mm fixed lens that has a f4 to an f 32 range... its all auto...why can't they put an f 2.0 on a lens like that????

PENTAX - smc PENTAX DA Star 300mm F4 ED(IF) SDM

I am not nessesarily talking about trying to get all creative with a 300mm prime lens...but maybe the 135mm and down range...

I know I jumped all over the place here but I am not a writer and don't really know how to frame my questions or thoughts so I am just writing what I can get down and hoping someone gets my ideas....

If in theory I could find a 100mm lens with a f2.0 I don't know why or if the focusing would be different...

Possible? not possible??? Please chime in....


Last edited by alamo5000; 06-05-2012 at 03:51 PM.
06-05-2012, 04:10 PM   #2
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Read this. It will answer all your current and future questions - but only if you read and process all that information.
06-05-2012, 04:21 PM   #3
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I will read it but I have been reading for like a year....now that I actually OWN a camera can you help me cut to the chase about my theories above?

The link is great and bookmarked but I am looking for discussion and to talk it over...as to my specific thoughts above....
06-05-2012, 04:27 PM   #4
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My above mentioned questions are more about visual effect in photos...I like science lessons but I also want to have a plain old 'english language' discussion on the larger aperture on a longer focal length and trying to create bokeh with that method....

In fact I have a series of questions...some of which are about the camera itself....if its about the amount of light hitting the sensor why can't you have an f4 at 300mm and then use a faster shutter speed???

06-05-2012, 04:33 PM - 1 Like   #5
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Shoot more and critically examine your photos. Go thru the discipline and discover what's it's all about on your own.
06-05-2012, 04:37 PM   #6
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QuoteOriginally posted by creampuff Quote
Shoot more and critically examine your photos. Go thru the discipline and discover what's it's all about on your own.
That's great and all but I can stare at the Mona Lisa for 2 hours and I still will only see some lady in the picture no matter how many hours I look at it.

There is no replacement for practicing but that's not getting me there. I need some assistance or a person who knows a helluva lot more than me to help me understand...

I need to get the jump start (or kick in the pants) so to say...
06-05-2012, 05:00 PM   #7
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QuoteOriginally posted by alamo5000 Quote
That's great and all but I can stare at the Mona Lisa for 2 hours and I still will only see some lady in the picture no matter how many hours I look at it.

There is no replacement for practicing but that's not getting me there. I need some assistance or a person who knows a helluva lot more than me to help me understand...

I need to get the jump start (or kick in the pants) so to say...
How many shots have you taken with your camera to date? Be honest? All of the top commercial shooters and very, very good serious enthusiasts that I personally know have put in literally thousands of shots to get their stellar keepers. It's no different with rank amateurs, the more you shoot the more keepers you'll get. The difference is in the percentages, the experienced will nail it more often simple as that.

So before you go NATO (no action, talk only), go out and shoot, not unthinking and randomly but with a clear defined thought process of what you want to achieve. Photography is a visual process but you need to experience it firsthand rather than being spoonfed with words on a forum. How do you translate head knowledge from a book, article or forum post into visual images which are not just technically good but aesthetically pleasing? You have to figure it out yourself.
06-05-2012, 05:19 PM   #8
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Every lens renders bokeh differently & there's no hard & fast rules about the relationship between aperture size & "quality" of OOF areas. Generally, "good" bokeh is achieved using the widest aperture the lens is capable of. This tends to produce nice round bokeh due to the wide-open nature of the lens. When stopping down, the bokeh can take on a hexagonal or other shape, depending on how many aperture blades the lens has. The problem with shooting wide-open all the time is that some lenses are sharper than others at max aperture. You need to experiment and see what works with your individual lenses.

I suggest taking a look here & pay attention to the lenses & apertures used in the examples: https://www.pentaxforums.com/forums/mini-challenges-games-photo-stories/93874...our-bokeh.html

The DA55-300 is called a variable-aperture zoom lens, usually meaning the aperture changes along with the focal length. Constant-aperture zoom lenses are a bit harder to find, but they are out there, and usually cost a bit more. The only one I own is a older Pentax M 75-150 f/4. So, as others here have said, get out there & take pictures & make some discoveries

06-05-2012, 05:21 PM - 1 Like   #9
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QuoteOriginally posted by alamo5000 Quote
...I don't know why--- but if you are at the 55mm zoom (not extended) the aperture will be allowed to go down to 4.0...but if you zoom out to 300mm it will not allow you to go down to 4.0. It will automatically shift to 5.8. Maybe its some auto setting? Is there some way to over ride this auto function so that I can get 4.0 at a 300mm zoom? I have tried a bunch of different settings and still...zoom to 300mm and the aperture is 5.8. ...
Not sure if you got this answered. The construction of the lens is such that when wide at 55mm the lens opening is widest at f4.0. When zoomed at 300mm the aperture will only open to what is computed to be f5.8 at its widest. It is a function of an economical design governed by physical limitations. You cannot override these constraints with the DA 55-300 lens design. Focal length and aperture have a linear relationship with this design. One of the resident mathematicians can go into detail because I cannot.

Last edited by lammie200; 06-07-2012 at 10:00 AM.
06-05-2012, 06:08 PM - 1 Like   #10
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QuoteOriginally posted by paulh Quote
Every lens renders bokeh differently & there's no hard & fast rules about the relationship between aperture size & "quality" of OOF areas. Generally, "good" bokeh is achieved using the widest aperture the lens is capable of. This tends to produce nice round bokeh due to the wide-open nature of the lens. When stopping down, the bokeh can take on a hexagonal or other shape, depending on how many aperture blades the lens has. The problem with shooting wide-open all the time is that some lenses are sharper than others at max aperture. You need to experiment and see what works with your individual lenses.
Can we maybe step away from this, since most of the time you aren't even are able to see the shape of the "blur disks" airy disk they are actually called i believe.
It's all about the lens elements and their rendering, nothing aperture blade can change about that.
Beside that we all know that even wide open lenses have different bokeh so claiming you get good bokeh when using the lens wide open is a bit puzzling to me where you get that from.


About depth of field, since that is what the OP is talking about.
If you want little DOF be sure to get high image magnification and low f-number.
So either or combination of close focusing, high focal length and/or low f-number would do the trick.


About f-number well it's a ratio between focal length and lens opening.
It's focal length divided by the iris opening
If you want f2 100mm lens the iris opening needs to be 50mm
if you want f4 100mm the iris opening is 25mm
So with your 300mm f/2 you need 150mm, you see the problem beside that the lens mount has a fixed diameter and that opening also needs to be taken into account.

With cheaper zooms the lens opening doesn't change enough to keep the f-ratio constant, when you zoom your lens from 100mm to 300mm the opening needs to be 3 times as large as well for example



here is a site that goes quite deep into depth of field, i hope you can manage to read it.
http://toothwalker.org/optics/dof.html

as for bokeh, that goes about how pleasing the blur is, it isn't about the amount or size but simply how pleasing it is.
Good bokeh is very pleasing and bad bokeh isn't pleasing, that's it in a nutshell.

Last edited by Anvh; 06-05-2012 at 06:21 PM.
06-06-2012, 04:55 AM   #11
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QuoteOriginally posted by alamo5000 Quote
I will read it but I have been reading for like a year....now that I actually OWN a camera can you help me cut to the chase about my theories above?
The link is great and bookmarked but I am looking for discussion and to talk it over...as to my specific thoughts above....
QuoteOriginally posted by alamo5000 Quote
My above mentioned questions are more about visual effect in photos...I like science lessons but I also want to have a plain old 'english language' discussion on the larger aperture on a longer focal length and trying to create bokeh with that method....
In fact I have a series of questions...some of which are about the camera itself....if its about the amount of light hitting the sensor why can't you have an f4 at 300mm and then use a faster shutter speed???
No quick solution for information, but I think it should be fine for us, LBA members:
The slow process of information gathering is helping our LBA development - it also slow down the acquisition process and therefore allow us have fund to live as normal persons.

My suggestion is going through each of your lenses - in details, analytic and systematical way, in multiple dimensions: f-stops, focal, distances (to subject, to background), light conditions, different background patterns - make notes, find out what is unique of each. So if people said 50/1.7 is 'better' than 50/1.4 - then why, and in what case it is better?

and to make photography more interesting - do not take photo of charts - spin your brain to find something more interesting to shot. Leave the charts and lens testing to professionals.

Going through information from others will save you alot of money and time, but only you know what to get and why.

Last edited by hoanpham; 06-06-2012 at 05:04 AM.
06-06-2012, 05:32 AM - 1 Like   #12
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i think a lot of the OP's questions are just from a lack of understanding of the aperture in general.

Just so it is clear, the aperture in F numbers is the focal length divided by the lens diameter. so for example he is asking about his 55-300 zoom, the reason it is F5.6 or 5.8 at 300mm is due to the diameter of the front element. that is the limiting factor. At shorter focal lengths some internal baffle is limiting the aperture and due to its position and diameter it is why there is a variable aperture as a function of focal length.

He asks about why there cant be a 100mmF2 lens, and note there are a few out there including a takumar 100/2, which, the last time I saw one on E-Bay had a $4000 BIN price

it is all about simple physics.,

Lenses are generally not designed for bokeh, they are designed for a ton of other reasons, and the bokeh is what ever is left.
06-06-2012, 05:59 AM - 1 Like   #13
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I think you have to learn about the relationship between focal length, aperture and depth of field (read!) and try out what you've learn. All the equipment won't make you a better photographer if you don't know your basics. It's much better to work with what you've got than keep thinking what you might need.
To get an idea what 'even' with a kit lens is possible: PENTAX : Select a PENTAX interchangeable lens camera or a lens model
06-06-2012, 07:43 AM   #14
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Two more good reads i suggest to you.

Understanding Camera Lenses
Understanding Depth of Field in Photography
06-06-2012, 11:06 AM - 1 Like   #15
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Ain't gonna comment on Bokeh - it's all subjective. Rest assured that something like your 55-300 will give you some very pleasing subject isolation.

Cor blimey, that link on "Equivalence" is heavy going, isn't it?

To answer your questions on "why isn't my 55-300 F4 at 300mm?" and so forth, you need to know a few things:
  1. Your F-stop is the focal length divided by the diameter of the entrance pupil (or aperture) of your lens
  2. The main purpose of a lens is to get light gathered by the front element through your lens mount without obnoxious optical defects screwing up your pictures
  3. This is more difficult at larger apertures
  4. This is more difficult at longer and shorter focal lengths relative to the size of your sensor (so harder for long telephoto and ultra wide)
  5. This is more difficult for zoom lenses
  6. Lens design is all about compromises between ideal performance and other important characteristics such as cost and weight.
So consider your (very nice, I might point out) zoom. Go to a website that sells camera equipment and look for lenses like 300mm F2.8 and 70-200mm F2.8. They're huge and expensive. So that's why your consumer zoom starts at F4: a 55-300 F2.8, if anyone sold such a beast, would go for about 10K and weigh about 5 kilos.

Now consider a constant aperture F4 zoom. This is desirable so that exposure doesn't change when you change focal length. A similar range of lengths is available with the DA* 60-250mm F4. It weighs over a kilo and costs over a grand. As does the DA* 300mm F4. So a 55-300mm F4 would both weigh and cost more than either of those two. Basically, that F5.8 at 300mm saves you about $1500 and about a kilo of weight. Good trade-off, I'd say.

If you do the "F-stop = focal length / entrance pupil diameter" arithmetic, you'll find that 5.8 @ 300mm corresponds to a diameter of 52mm, give or take. You may well ask "why can't my zoom be F1.1 @ 55mm?". To answer that, see points two and three above. F1.1 on a consumer zoom (or a pro-grade zoom for that matter) would result in pictures that looked like sh*t.

You should be able to extend this to "why not F2 for a 300mm lens", etc.


Other stuff:

Yes, you can get a very fast 135mm lens for K-mount. It's the A* 135mm F1.8.

A macro lens is one that's designed to focus very close-up and offer good performance right across the frame. That again is difficult to achieve which is why you generally don't find macro lenses faster than F2.8.*

If you don't like auto-everything, no-one's forcing you. You can easily set everything to manual if you so wish. When you come to use that 85mm @ F1.4 you may appreciate auto-focus at the very least! I'd say your camera's a means to an end and the easier it is to use the better. Framing is a way more important skill than being able to calculate exposure using the sunny 16 rule in your head. Just my opinion, though.


*Yes, I know that Olympus do an F2 macro lens for four thirds. They also do a couple of F2 fast zooms. I could have added a point above "This is somewhat easier for smaller sensors".
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