greetings....recently sourced some new trays and installed ventillation so I could print larger 16x20 prints. i am using a 75mm lens which i know is not optimal and was prepared for soft, under exposed corners on the maiden voyage. however, i'm noticing a burn close to center which I've diagnosed as as a result of the focus being so recessed on the enlarger bellows that it results in uneven light image/ orb. if i extend the bellows without the negative in place, i noticethelight become very even again. is that what you would expect? will a 105mm do the trick? or does my 23c have another issue? i am assuming longer focal length will mean backing up the column further but will it also mean extending the bellows for focus to eliminate the prob? thx, am interested in your experiences.